Low noise nulling circuit, nulling method and capacitive displacement sensor

Abstract

The application discloses a low-noise zero adjustment circuit, a zero adjustment method and a capacitive displacement sensor, and belongs to the technical field of capacitive displacement sensor signal processing. The circuit comprises a signal source, a probe, a digital-to-analog converter, an adder, a preamplification circuit, a rectification filter circuit, an analog-to-digital converter and a microcontroller. The probe modulates an alternating excitation signal to generate an initial displacement voltage signal, the digital-to-analog converter generates an inverted compensation voltage signal with a homologous alternating excitation signal, the two signals are inverted and superposed by the adder to offset a base signal component and a homologous excitation noise, and then the digital-to-analog converter generates a digital displacement signal through amplification, rectification filtering and analog-to-digital conversion, and the microcontroller feeds back and adjusts the compensation voltage to complete zero adjustment. The application offsets large base signals and related noises in an analog domain, expands an effective relative displacement measurement range on both sides of a zero adjustment position, and improves noise consistency and measurement resolution at a large displacement.

Description

Technical Field

[0001] This invention relates to the field of signal processing technology for capacitive displacement sensors, and in particular to a low-noise zeroing circuit, a zeroing method, and a capacitive displacement sensor. Background Technology

[0002] Capacitive displacement sensors offer advantages such as high measurement accuracy, non-contact measurement, and fast dynamic response, making them widely used in precision measurement applications such as advanced semiconductor equipment, precision machine tool spindle vibration detection, ultra-precision machining, and metrology. Capacitive displacement sensors obtain the distance between the probe and the target surface by detecting the capacitance formed between them.

[0003] When a capacitive displacement sensor operates, an AC excitation signal must be applied. The probe modulates the AC excitation signal, converting the displacement information between the probe and the target surface into an AC voltage signal As. This AC voltage signal is then rectified and filtered to convert into a DC analog voltage Ds. The magnitude of the DC analog voltage Ds is then read by an analog-to-digital converter (ADC), and finally, the displacement is calculated by a microcontroller (MCU). Figure 1 As shown.

[0004] The aforementioned measurement principle dictates that the greater the displacement between the probe and the target surface, the greater the amplitude of the AC voltage signal As output by the sensor. Since the noise in As primarily originates from the signal source, and the signal-to-noise ratio of the signal source is essentially constant, this means that the greater the amplitude of the output signal, the greater the absolute noise it carries; in other words, the greater the displacement, the greater the sensor noise. Therefore, the sensor exhibits high resolution at small displacements, while its resolution significantly decreases at large displacements.

[0005] In most measurement scenarios, users are primarily concerned with the relative displacement of the target surface relative to the probe, rather than the absolute displacement, thus creating a need for zeroing. The current mainstream zeroing solution is to not perform zeroing on the analog signal, but directly use an ADC to convert the analog voltage into a digital signal, and then have the MCU subtract the digital signal to zero in the digital domain before outputting it.

[0006] While the existing solutions described above can achieve the zeroing function, they have the following drawbacks: 1. The problems of high noise and low resolution at large displacements remain unresolved. The voltage output of a capacitive displacement sensor is proportional to the displacement, and its noise also increases with the displacement, leading to a decrease in resolution at large displacements. Existing digital domain zeroing methods can only bring the digital output to zero at large displacements, but cannot reduce the noise that increases with displacement, meaning they cannot maintain the same noise level at large displacements as at small displacements.

[0007] 2. Limited effective zeroing range and biased zeroing position. The capacitive displacement sensor has a limited range, and the distance between the probe and the target surface is approximately proportional to the output analog voltage. Since existing solutions do not cancel the base signal in the analog domain, when the probe is at a large displacement position, the upper limit of the signal amplitude that the operational amplifier can handle limits the large value of the base signal output by the probe, which can easily cause the preamplifier circuit to approach or enter saturation. At this time, the probe can often only move in the direction of decreasing displacement, making it difficult to achieve symmetrical relative displacement measurement on both sides of the zeroing position.

[0008] Therefore, how to suppress noise at large displacements while achieving relative displacement zeroing and expanding the effective relative displacement measurement range on both sides of the zeroing position is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] To address the aforementioned issues, this invention provides a low-noise zeroing circuit, a zeroing method, and a capacitive displacement sensor. By canceling large-base signals and related noise, it expands the effective relative displacement measurement range symmetrical on both sides of the zeroing position, and improves noise consistency and measurement resolution at large displacements.

[0010] The above objectives can be achieved through the following approach: A low-noise zero-adjustment circuit includes a signal source for generating an AC excitation signal; a probe connected to the signal source for receiving the AC excitation signal and performing a modulation operation to generate an initial displacement voltage signal; a digital-to-analog converter connected to the signal source for acquiring the AC excitation signal and performing a digital-to-analog conversion voltage division operation on the AC excitation signal to generate a compensation voltage signal, wherein the compensation voltage signal is out of phase with the initial displacement voltage signal, and the amplitude and excitation noise carried by the compensation voltage signal are proportionally scaled to the AC excitation signal and originate from the same source as the excitation noise in the initial displacement voltage signal; and an adder connected to both the probe and the digital-to-analog converter for performing inverse superposition processing on the initial displacement voltage signal and the compensation voltage signal to cancel the initial displacement voltage signal. The base signal component corresponding to the absolute displacement in the pressure signal and the excitation noise it carries are used to generate a composite voltage signal that retains the relative displacement change. A preamplifier circuit, connected to the adder, is used to amplify the composite voltage signal to generate an amplified composite signal. A rectifier and filter circuit, connected to the preamplifier circuit, is used to rectify and filter the amplified composite signal to generate a DC analog voltage signal. An analog-to-digital converter, connected to the rectifier and filter circuit, is used to perform analog-to-digital conversion on the DC analog voltage signal to generate a digital displacement signal. A microcontroller, connected to both the analog-to-digital converter and the digital-to-analog converter, is used to generate a feedback adjustment command using the digital displacement signal and send the feedback adjustment command to the digital-to-analog converter to update the compensation voltage signal, thereby completing the zero-adjustment control.

[0011] Optionally, the digital-to-analog converter (DAC) is a multiplicative DAC, wherein the reference signal input terminal of the multiplicative DAC receives the AC excitation signal, and the digital control terminal receives the voltage divider control parameters output by the microcontroller; or the DAC is combined with an inverting amplifier circuit to convert the in-phase scaling signal output by the DAC into a compensation voltage signal that is out of phase with the initial displacement voltage signal; the DAC includes any one of a voltage output DAC, a digital potentiometer, a programmable gain amplifier, or a switched resistor array.

[0012] Optionally, the rectifier and filter circuit is a phase-sensitive rectifier circuit that is in phase and frequency with the AC excitation signal, or a synchronous detector, a lock-in amplifier, or a digital demodulation circuit; the microcontroller includes a digital signal processor or a field-programmable gate array.

[0013] Optionally, the microcontroller is used to determine whether the digital displacement signal is within a preset target intermediate position range and generate the feedback adjustment command accordingly. The target intermediate position range is set at the middle position of the input range of the analog-to-digital converter, so that the digital displacement signal is located within the target intermediate position range after zeroing, and a symmetrical effective relative displacement measurement range is maintained on both sides of the target intermediate position range. The microcontroller is also used to lock the output state of the digital-to-analog converter after zeroing to fix the compensation voltage signal.

[0014] Based on the same inventive concept, this invention also provides a low-noise zero-adjustment method, employing the zero-adjustment circuit described above. The method includes: acquiring an AC excitation signal and performing a modulation operation on the AC excitation signal to generate an initial displacement voltage signal; acquiring the AC excitation signal and performing a digital-to-analog conversion voltage divider operation on the AC excitation signal to generate a compensation voltage signal, wherein the compensation voltage signal is out of phase with the initial displacement voltage signal, and the amplitude and excitation noise carried by the compensation voltage signal are proportionally scaled to the AC excitation signal and originate from the same source as the excitation noise in the initial displacement voltage signal; performing an inverse superposition process on the initial displacement voltage signal and the compensation voltage signal to cancel the base signal component corresponding to the absolute displacement and the excitation noise carried by the initial displacement voltage signal, generating a composite voltage signal that retains the relative displacement change; performing a signal conditioning operation on the composite voltage signal to generate a DC analog voltage signal; performing an analog-to-digital conversion operation on the DC analog voltage signal to generate a digital displacement signal; generating a feedback adjustment command using the digital displacement signal, and updating the compensation voltage signal according to the feedback adjustment command, thereby completing the zero-adjustment control.

[0015] Optionally, acquiring the AC excitation signal and performing a digital-to-analog conversion voltage division operation on the AC excitation signal to generate a compensation voltage signal includes: connecting the AC excitation signal to the reference input terminal of the digital-to-analog converter, so that the output of the digital-to-analog converter simultaneously contains the effective component of the AC excitation signal and the corresponding homogeneous excitation noise; adjusting the voltage division ratio of the digital-to-analog conversion using the feedback adjustment command or a preset voltage division parameter to generate voltage division control parameters; using the voltage division control parameters to perform amplitude scaling processing on the AC excitation signal, and inverting the signal to generate a compensation voltage signal with a phase opposite to the initial displacement voltage signal; wherein the amplitude of the compensation voltage signal and the excitation noise it carries are both proportionally scaled from the AC excitation signal.

[0016] Optionally, the step of superimposing the initial displacement voltage signal and the compensation voltage signal to generate a composite voltage signal includes: extracting the base signal component from the initial displacement voltage signal and the compensation signal component from the compensation voltage signal, wherein the base signal component and the compensation signal component contain excitation noise from the same source; adding the initial displacement voltage signal and the compensation voltage signal in opposite phases, so that the base signal component and the compensation signal component cancel each other out, and the excitation noise from the same source carried by the base signal component and the compensation signal component cancels each other out, generating a noise-reduced composite signal that retains the carrier component corresponding to the relative displacement change and the residual noise as the composite voltage signal.

[0017] Optionally, the step of generating a feedback adjustment command using the digital displacement signal and updating the compensation voltage signal according to the feedback adjustment command includes: determining whether the digital displacement signal is within a preset target intermediate position range, generating a position determination result, wherein the target intermediate position range is set at the middle position of the input range of the analog-to-digital converter, and symmetrical relative displacement measurement ranges are maintained on both sides based on the center value of the target intermediate position range; generating a feedback adjustment command according to the position determination result; and sending the feedback adjustment command to the digital-to-analog converter to update the amplitude of the compensation voltage signal, so that the digital displacement signal is located within the target intermediate position range, thereby forming a symmetrical effective relative displacement measurement range on both sides of the zero-adjustment position.

[0018] Optionally, the step of sending the feedback adjustment command to the digital-to-analog converter to update the amplitude of the compensation voltage signal so that the digital displacement signal is located in the target's middle position range further includes: locking the output state of the digital-to-analog converter to fix the compensation voltage signal; using the digital displacement signal located in the target's middle position range as a zero-adjustment reference, acquiring the relative displacement changes generated by the probe moving away from the target surface and moving closer to the target surface, respectively, measuring the symmetrical relative displacement changes on both sides of the target's middle position range, and generating a relative displacement measurement result with direction information.

[0019] Based on the same inventive concept, the present invention also provides a capacitive displacement sensor employing the zeroing method described above, including a target surface and a low-noise zeroing circuit; the target surface is made of a conductive material; the low-noise zeroing circuit includes a signal source, a probe, a digital-to-analog converter, an adder, a preamplifier circuit, a rectifier and filter circuit, an analog-to-digital converter, and a microcontroller; the probe includes mutually insulated measuring electrodes and shielding electrodes, a capacitor is formed between the measuring electrodes and the target surface, and the shielding electrodes are used to limit the diffusion of the edge electric field, so that the capacitance change is mainly caused by the change in distance between the probe and the target surface.

[0020] Compared with the prior art, the present invention has the following advantages: 1. This invention utilizes a digital-to-analog converter to generate a compensation voltage signal that is inversely related to the initial displacement voltage signal, using the same AC excitation signal as a reference. The two signals are then superimposed in inverse phase before preamplification and rectification filtering, thereby canceling the large base signal component corresponding to the absolute displacement in the analog domain. This allows the zero-adjusted signal to return to the middle operating range of the subsequent circuit. Even if the probe is located at a large range position, it is not easy for the preamplifier circuit to saturate, thus expanding the effective relative displacement measurement range symmetrical on both sides of the zero-adjustment position. 2. The compensation voltage signal of this invention is derived from the AC excitation signal through a controllable scaling ratio. Its amplitude and the excitation noise it carries are from the same source as the base signal component in the initial displacement voltage signal, without introducing additional noise. When the two are superimposed in opposite phases, the relevant excitation noise carried by the large base signal is canceled out. The synthesized signal retains only the carrier component corresponding to the relative displacement change and the residual noise, thereby improving the consistency of the noise level and the measurement resolution at large displacements across the entire range. 3. This invention achieves automatic zeroing by adjusting the input code of the digital-to-analog converter through a microcontroller closed-loop adjustment, and locks the output to a fixed reference after zeroing. It can measure the relative displacement of the probe in both directions with directional information. Moreover, it is not limited to capacitive displacement sensors. The digital-to-analog converter and rectifier filter circuit can be implemented in various equivalent forms, which is convenient for engineering applications. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the principle of prior art zeroing in an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of a low-noise zero-adjustment circuit using bias signal subtraction according to an embodiment of the present invention.

[0023] Figure 3 This is a flowchart illustrating a low-noise zeroing method using bias signal subtraction according to an embodiment of the present invention.

[0024] Figure 4 This is a flowchart of the zero-adjustment feedback control according to an embodiment of the present invention.

[0025] Figure 5 This is a schematic diagram of signal waveform and noise cancellation according to an embodiment of the present invention. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] This embodiment provides a low-noise zero-adjustment circuit. This circuit is primarily used in capacitive displacement sensors, but can also be applied to other sensors that modulate and output an AC signal related to the measured quantity using a homogeneous AC excitation signal.

[0028] like Figure 2 As shown, the low-noise zeroing circuit of this embodiment includes a signal source, a probe, a digital-to-analog converter, an adder, a preamplifier circuit, a rectifier and filter circuit, an analog-to-digital converter, and a microcontroller.

[0029] The signal source is connected to both the probe and the digital-to-analog converter to generate an AC excitation signal. The AC excitation signal is preferably a periodic AC signal, such as a sinusoidal signal. The frequency and amplitude of the AC excitation signal are determined based on the probe's structure, measurement range, and the operating range of the signal conditioning circuit.

[0030] The probe receives the AC excitation signal and senses the capacitance formed between the probe and the target surface. The probe modulates the AC excitation signal based on this capacitance to generate an initial displacement voltage signal. The amplitude of the initial displacement voltage signal has a predetermined correspondence with the distance between the probe and the target surface.

[0031] The digital-to-analog converter (DAC) is connected to both the signal source and the microcontroller. It scales the AC excitation signal from the signal source in an adjustable manner based on the control parameters output by the microcontroller and outputs a compensation voltage signal. The compensation voltage signal has the same or corresponding frequency as the initial displacement voltage signal, and is out of phase with the initial displacement voltage signal at the superposition node of the adder.

[0032] In one implementation, the digital-to-analog converter (DAC) is a multiplicative DAC. Its reference signal input receives the AC excitation signal, and its digital control input receives voltage divider control parameters output by a microcontroller, thereby scaling the AC excitation signal according to an adjustable ratio. In another implementation, a combination of the DAC and an inverting amplifier circuit can be used to convert the non-inverting scaled signal output by the DAC into an inverted signal. Differential output signal sources, programmable gain circuits, digital potentiometers, or controllable resistor networks can also be used to achieve the same function.

[0033] The adder is connected to both the probe and the digital-to-analog converter to superimpose the initial displacement voltage signal and the compensation voltage signal. Since the two signals are out of phase at the superposition node, the superposition is essentially a subtraction operation. The adder outputs the synthesized voltage signal.

[0034] The preamplifier circuit is connected to the adder to amplify the synthesized voltage signal, so as to make full use of the input dynamic range of the subsequent rectifier and filter circuit and analog-to-digital converter.

[0035] The rectifier and filter circuit is connected to the preamplifier circuit to rectify and filter the amplified composite voltage signal, converting the amplitude information of the AC signal into a DC analog voltage signal.

[0036] The analog-to-digital converter is connected to the rectifier and filter circuit to sample and quantize the DC analog voltage signal to generate a digital displacement signal.

[0037] The microcontroller is connected to both the analog-to-digital converter (ADC) and the digital-to-analog converter (DAC). The microcontroller reads the digital displacement signal, generates a feedback adjustment command based on the relationship between the digital displacement signal and the preset target midpoint position range, and uses this command to control the output amplitude of the DAC. After zeroing is complete, the microcontroller locks the output state of the DAC to keep the compensation voltage signal constant.

[0038] Reference Figure 3 One embodiment of the present invention proposes a low-noise zeroing method, which expands the effective relative displacement measurement range symmetrical on both sides of the zeroing position by canceling large substrate signals and related noise, and improves noise consistency and measurement resolution at large displacements.

[0039] The method in this embodiment includes steps S1-S7: S1. Acquire the AC excitation signal and use the AC excitation signal to perform modulation operation to generate an initial displacement voltage signal; Specifically, the AC excitation signal is generated by a low-distortion sinusoidal signal source. After being buffered, the AC excitation signal is simultaneously input to the probe and the digital-to-analog converter (DAC). The buffer isolates the probe load from the DAC load, preventing load changes from causing amplitude variations in the AC excitation signal. The frequency of the AC excitation signal should be higher than the highest frequency of the measured mechanical displacement change, but lower than the effective bandwidth of the probe and subsequent circuitry. In this embodiment, the AC excitation signal frequency is set to at least 20 times the highest frequency of the measured mechanical displacement change. This multiple is determined based on calibration results from over 200 frequency scans, ensuring that changes in mechanical displacement are reflected as changes in the AC excitation signal amplitude, while avoiding significant phase shifts in the probe connection lines. The AC excitation signal is represented as: , In the formula, For a moment The corresponding AC excitation signal, in volts; The peak value of the AC excitation signal is expressed in volts. The frequency of the AC excitation signal is expressed in Hertz (Hz). Time, in seconds; The time-varying excitation noise in the AC excitation signal is expressed in volts.

[0040] The probe consists of mutually insulated measuring electrodes and shielding electrodes, while the target surface is made of a conductive material. The measuring electrodes and the target surface form a capacitance. The shielding electrodes limit the diffusion of the edge electric field, ensuring that capacitance changes are primarily caused by variations in the distance between the probe and the target surface. When the probe is parallel to the target surface, the capacitance can be determined using the following formula: , In the formula, This refers to the capacitance formed between the probe and the target surface, measured in farads. The vacuum permittivity is expressed in farads per meter. It is the relative permittivity of the medium between the probe and the target surface, and is a dimensionless quantity; The effective overlap area between the electrode and the target surface is measured in square meters. This is the distance between the probe and the target surface, in meters.

[0041] The sensing circuit inputs an AC excitation signal to the probe and detects the AC current flowing through the probe capacitor. The sensing circuit employs a transimpedance converter structure to convert the AC current into voltage. Because the probe capacitor, cable parasitic capacitance, and edge electric field can cause fixed deviations, at least 20 equally spaced calibration positions are set within the complete measurement range before actual use, and at least 200 capacitance measurements are collected at each calibration position. The arithmetic mean of the capacitance measurements at each calibration position is taken, and then a correspondence between capacitance and displacement information is established. The displacement information represents the distance between the probe and the target surface, or the change in distance relative to the calibration starting point.

[0042] Within the probe's effective measurement range, the correspondence between capacitance and displacement information can be established using a piecewise linear lookup table. Displacement information between adjacent calibration positions is calculated using the following formula: , In the formula, This is displacement information, in meters; and The known distance between adjacent calibration positions, in meters; The capacitance currently sensed is expressed in farads. and This represents the average capacitance value corresponding to adjacent calibration positions, in farads.

[0043] The modulation circuit changes the amplitude of the AC excitation signal based on the displacement information, but does not change the frequency of the AC excitation signal. The amplitude modulation coefficient of the modulation circuit is determined by the aforementioned calibration data. When using linear calibration within the effective range, the initial displacement voltage signal is expressed as: , In the formula, This is the initial displacement voltage signal, measured in volts. The amplitude modulation coefficient corresponding to the reference position is a dimensionless quantity. This is the calibration slope between displacement information and amplitude modulation coefficients, expressed in meters. This is the current displacement information, in meters; This provides displacement information corresponding to the reference position, in meters. The excitation signal is measured in volts.

[0044] For example, the effective overlap area of ​​the measuring electrodes is set to The area is square meters, the space between the probe and the target surface is air, the relative permittivity is set to 1, and the distance between the probe and the target surface is set to... If the meter is [value], then the capacitance is [value]. The peak value of the AC excitation signal is set to 1 volt, and the frequency is set to 100 kHz. The amplitude modulation coefficient corresponding to the reference position is set to 0.600, and the calibration slope is set to 800 per meter. When the probe is in the reference position, the peak value of the initial displacement voltage signal is... When the distance between the probe and the target surface increases by 10 micrometers, the amplitude modulation coefficient is... The peak value of the initial displacement voltage signal then becomes This completes the conversion from capacitance value and displacement information to the initial displacement voltage signal.

[0045] S2. Acquire the AC excitation signal and perform digital-to-analog conversion voltage division operation on the AC excitation signal to generate a compensation voltage signal, wherein the compensation voltage signal is out of phase with the initial displacement voltage signal, and the amplitude and excitation noise carried by the compensation voltage signal are scaled proportionally to the AC excitation signal and are from the same source as the excitation noise in the initial displacement voltage signal. In one embodiment of the present invention, step S2 includes the following steps: The AC excitation signal is connected to the reference input terminal of the digital-to-analog converter, so that the output of the digital-to-analog converter simultaneously contains the effective component of the AC excitation signal and the corresponding homogeneous excitation noise. The voltage division ratio of the digital-to-analog converter is adjusted using the feedback adjustment command or the preset voltage division parameter to generate voltage division control parameters; The AC excitation signal is amplitude scaled using the voltage divider control parameters and then inverted to generate a compensation voltage signal that is out of phase with the initial displacement voltage signal. The amplitude of the compensation voltage signal and the excitation noise it carries are both scaled proportionally to the AC excitation signal.

[0046] Specifically, the digital-to-analog converter (DAC) receives an AC excitation signal and modifies its amplitude using a programmable resistor divider network. The DAC's reference input is connected to the AC excitation signal, ensuring that the DAC output simultaneously contains the effective component of the AC excitation signal and the excitation noise from the same source. The DAC output is connected to an inverting buffer circuit, which flips the phase by 180°, thus making the compensation voltage signal out of phase with the initial displacement voltage signal.

[0047] The digital-to-analog converter (DAC) uses preset voltage divider parameters before zeroing begins. After zeroing begins, a feedback adjustment command overrides the preset voltage divider parameters and changes the DAC's digital input code. The voltage divider control parameters are jointly determined by the digital input code, the DAC's bit depth, and the gain of the inverting buffer circuit. , In the formula, These are voltage divider control parameters, which are dimensionless quantities. The amplitude gain of the inverting buffer circuit is a dimensionless quantity. This is the digital input code for the digital-to-analog converter, and it belongs to the dimensionless integer category. The bit depth of the digital-to-analog converter is a dimensionless integer. The AC excitation signal is amplitude-scaled using voltage divider control parameters, and a compensation voltage signal is generated through an inverting buffer circuit. , In the formula, The unit is volts, used to compensate for voltage signals. These are voltage divider control parameters, which are dimensionless quantities. The AC excitation signal is measured in volts; the negative sign indicates that the compensation voltage signal is out of phase with the AC excitation signal.

[0048] The output update of the digital-to-analog converter should be performed near the zero-crossing moment of the AC excitation signal, or the current output code should be maintained during the update to avoid pulse interference caused by output code switching. The compensation voltage signal should only be read in step S3 after the digital-to-analog converter output has stabilized.

[0049] For example, the digital-to-analog converter is set to 16 bits, the amplitude gain of the inverting buffer circuit is set to 1, and the digital input code is set to 39321. The voltage divider control parameters are calculated as follows: When the peak value of the AC excitation signal is 1 volt, the compensation voltage signal is: The compensation voltage signal has the same amplitude and opposite phase as the initial displacement voltage signal corresponding to the reference position.

[0050] S3. The initial displacement voltage signal and the compensation voltage signal are superimposed in opposite phase to cancel the base signal component corresponding to the absolute displacement and the excitation noise carried in the initial displacement voltage signal, and generate a composite voltage signal that retains the relative displacement change. In one embodiment of the present invention, step S3 includes the following steps: Extract the base signal component from the initial displacement voltage signal and the compensation signal component from the compensation voltage signal, wherein the base signal component and the compensation signal component contain excitation noise from the same source; The initial displacement voltage signal and the compensation voltage signal are added in opposite phases, so that the base signal component and the compensation signal component cancel each other out, and the common source excitation noise carried by the base signal component and the compensation signal component cancel each other out, generating a noise-reduced synthetic signal that retains the carrier component corresponding to the relative displacement change and the residual noise as the synthetic voltage signal.

[0051] Specifically, the initial displacement voltage signal is input to the in-phase input channel of the adder circuit, and the compensation voltage signal is input to the bias input channel of the adder circuit. Both input channels use matching resistors with a resistance error not exceeding 0.01%, and the propagation delay difference between the two input channels is ensured to be no more than one-thousandth of the AC excitation signal period, reducing the cancellation residuals caused by amplitude and phase errors.

[0052] The base signal component in the initial displacement voltage signal represents the excitation carrier component corresponding to the reference position. The compensation signal component in the compensation voltage signal represents the inverted signal component generated by the digital-to-analog converter based on the AC excitation signal. Both the base signal component and the compensation signal component originate from the same AC excitation signal and therefore contain excitation noise from the same source. The adder circuit adds the initial displacement voltage signal and the compensation voltage signal in inverted phase to obtain: , In the formula, This is a synthesized voltage signal, measured in volts. This is the initial displacement voltage signal, measured in volts. The unit is volts, used to compensate for voltage signals.

[0053] Substituting the signal relationships from steps S1 and S2, we get: , In the formula, The amplitude modulation coefficient corresponding to the reference position is a dimensionless quantity. This is the calibration slope between displacement information and amplitude modulation coefficients, expressed in meters. This is the current displacement information, in meters; This provides displacement information corresponding to the reference position, in meters. These are voltage divider control parameters, which are dimensionless quantities. The excitation signal is measured in volts. The synthesized voltage signal is expressed in volts. When the voltage divider control parameters are equal to the amplitude modulation coefficient corresponding to the reference position, the synthesized voltage signal simplifies to: , In the formula, This is a synthesized voltage signal, measured in volts. To calibrate the slope, the unit is per meter; This represents the relative displacement change, expressed in meters. The excitation signal is measured in volts.

[0054] The base signal component and the excitation noise carried by the base signal component are canceled by the compensation signal component. The synthesized voltage signal retains only the carrier component corresponding to the relative displacement change and the residual noise that is not completely canceled, thus forming a noise-reduced synthesized signal.

[0055] For example, the initial displacement voltage signal corresponding to the reference position is 0.600 volts, the compensation voltage signal is -0.600 volts, and the composite voltage signal at the reference position is calculated as follows: When the distance between the probe and the target surface increases by 10 micrometers, the initial displacement voltage signal is 0.608 volts, the compensation voltage signal remains at -0.600 volts, and the synthesized voltage signal is calculated as follows: When the peak excitation noise is 1 mV, the initial displacement voltage signal corresponding to the reference position carries 0.600 mV of excitation noise, and the compensation voltage signal carries -0.600 mV of excitation noise; the reference noises cancel each other out. The residual homogeneous excitation noise after increasing the distance by 10 micrometers is calculated as follows: The calculation results show that the noise from the same source in the denoised synthesized signal is reduced from 0.600 mV corresponding to the reference position to 0.008 mV corresponding to the relative displacement change.

[0056] S4. Perform signal conditioning operation on the synthesized voltage signal to generate a DC analog voltage signal; In one embodiment of the present invention, step S4 includes the following steps: The synthesized voltage signal is pre-amplified to generate an amplified synthesized signal; The amplified and synthesized signal is rectified and filtered to generate a DC analog voltage signal.

[0057] Specifically, the synthesized voltage signal is input to a preamplifier circuit. The preamplifier circuit uses a low-noise operational amplifier and a matching feedback resistor. The amplification factor is determined based on the maximum relative displacement change, the zero-adjustment range of the digital-to-analog converter (DAC), and the input range of the DAC. The maximum peak value after amplification should be less than the linear output range of the operational amplifier, and a voltage margin of at least 10% should be reserved for the subsequent DC analog voltage signal. The amplified and synthesized signal is generated according to the following formula: , In the formula, To amplify the synthesized signal, the unit is volts; This is the voltage gain of the preamplifier circuit, which is a dimensionless quantity. This is a synthesized voltage signal, measured in volts.

[0058] The rectification and filtering process can employ a sinusoidal phase-sensitive rectification method with the same frequency and phase as the AC excitation signal. Phase-sensitive rectification multiplies the amplified composite signal with a sinusoidal reference signal of the same frequency and phase, converting the amplified composite signal in phase with the AC excitation signal into a positive DC component, and converting the amplified composite signal out of phase with the AC excitation signal into a negative DC component. Phase-sensitive rectification retains the directional information corresponding to the probe's movement towards or from a distance. The phase-sensitive rectified output is superimposed with the midpoint voltage of the analog-to-digital converter input to prevent the negative polarity signal from exceeding the input range of the analog-to-digital converter. The DC analog voltage signal is formed according to the following formula: , In the formula, This is a DC analog voltage signal, measured in volts. The input midpoint voltage of the analog-to-digital converter, in volts; This indicates low-pass filtering. To amplify the synthesized signal, the unit is volts; The frequency of the AC excitation signal is expressed in Hertz (Hz). The time is expressed in seconds; coefficient 2 is used to compensate for the average coefficient after multiplying the sinusoidal reference signal, and is a dimensionless quantity. The low-pass filtering process uses a resistor-capacitor filter network, and the cutoff frequency is determined by the following formula: , In the formula, This is the low-pass filter cutoff frequency, measured in Hertz. This is the filter resistor, measured in ohms. This is the filter capacitor, measured in farads. The low-pass filter cutoff frequency is set to less than one-hundredth of the AC excitation signal frequency and higher than the highest frequency of change of the measured mechanical displacement, balancing carrier suppression and displacement tracking capabilities.

[0059] For example, the synthesized voltage signal is 0.008 volts, and the voltage gain of the preamplifier circuit is set to 20. The amplified synthesized signal is calculated as follows: The analog-to-digital converter (ADC) input range is 0 volts to 5 volts, the ADC input midpoint voltage is set to 2.5 volts, and the DC analog voltage signal after phase-sensitive rectification is calculated as follows: The AC excitation signal frequency is 100 kHz, the filter resistor is set to 15.9 kΩ, and the filter capacitor is set to 100 nanofarads. The low-pass filter cutoff frequency is calculated as follows: The 100.1 Hz cutoff frequency is less than one percent of the 100 kHz AC excitation signal frequency, which can filter out the high-frequency components generated by rectification.

[0060] S5. Perform analog-to-digital conversion on the DC analog voltage signal to generate a digital displacement signal; Specifically, the analog-to-digital converter (ADC) receives a DC analog voltage signal and uses a stable reference voltage for sampling and quantization. The ADC's input range covers the lowest and highest voltages that the DC analog voltage signal can reach. The ADC sampling frequency is set to at least 10 times the low-pass filter cutoff frequency, allowing continuous recording of changes in the DC analog voltage signal. The digital quantization code corresponding to the DC analog voltage signal is determined according to the following formula: , In the formula, For the first The digital quantization code corresponding to each sample is a dimensionless integer. This indicates taking the nearest integer; The number of bits in the analog-to-digital converter is a dimensionless integer. For the first The DC analog voltage signal corresponding to each sample is in volts; This is the reference voltage for the analog-to-digital converter, measured in volts.

[0061] To reduce quantization jumps and residual circuit noise, a fixed number of digital quantization codes are continuously acquired and their arithmetic mean is calculated. The fixed number is set based on the standard deviation of 200 static measurements, ensuring that the standard deviation of the averaged digital quantization code does not exceed one-quarter of the width of the target midpoint interval. The digital displacement signal is generated according to the following formula: , In the formula, It is a digital displacement signal, measured in bits; The number of samples used in the averaging process is a dimensionless integer. For the first The digital quantization code corresponding to each sample is expressed in bits. After summing and dividing by the number of samples, the digital displacement signal is still represented in bits.

[0062] When the analog-to-digital converter outputs full-scale or zero-code data, the controller marks the current digital displacement signal as out-of-range data and does not use the out-of-range data to generate feedback adjustment commands, thus preventing saturation signals from causing incorrect adjustments.

[0063] For example, the analog-to-digital converter is set to 16 bits, the reference voltage is set to 5 volts, and the DC analog voltage signal is 2.660 volts. The digital quantization code is... When the input midpoint voltage of the analog-to-digital converter is 2.5 volts, the midpoint digital quantization code is calculated as follows: When all 64 consecutive samples remain stable between 34864 and 34866 codes, the sum of these 64 samples is divided by 64 to obtain the digital displacement signal. Assuming 32 samples yield 34864 codes, 24 samples yield 34865 codes, and 8 samples yield 34866 codes, the digital displacement signal is calculated as follows: The controller retains decimal results for feedback calculations and relative displacement calculations.

[0064] S6. Generate a feedback adjustment command using the digital displacement signal, and update the compensation voltage signal according to the feedback adjustment command, thereby completing the zero-adjustment control.

[0065] In one embodiment of the present invention, step S6 includes the following steps: Determine whether the digital displacement signal is within a preset target middle position range, and generate a position determination result. The target middle position range is set at the middle position of the input range of the analog-to-digital converter, and a symmetrical relative displacement measurement range is maintained on both sides based on the center value of the target middle position range. Based on the position determination result, a feedback adjustment command is generated; The feedback adjustment command is sent to the digital-to-analog converter to update the amplitude of the compensation voltage signal, so that the digital displacement signal is located in the middle position range of the target, thereby forming a symmetrical effective relative displacement measurement range on both sides of the zero adjustment position.

[0066] Specifically, the target midpoint position interval is set near the middle of the analog-to-digital converter (ADC) input range. The center value of the target midpoint position interval uses the midpoint digital quantization code, and the interval half-width is determined based on the standard deviation of 200 digital displacement signals under static conditions. The interval half-width is not less than four times the standard deviation and does not exceed 1% of the ADC's full-scale code, thus avoiding noise-induced repeated adjustments by the ADC while preserving the symmetrical measurement range required for probe movement to varying distances. The position determination result is generated according to the following formula: , In the formula, The center value of the target's middle position interval, in code; The half-width of the target's middle position interval, in bytes; This is a digital displacement signal, measured in bits. All quantities are in bits. When the digital displacement signal is within the target's midpoint range, the position determination indicates that zeroing is complete. When the digital displacement signal is above the target's midpoint range, the feedback adjustment command increases the digital input bit of the digital-to-analog converter, increasing the amplitude of the compensation voltage signal. When the digital displacement signal is below the target's midpoint range, the feedback adjustment command decreases the digital input bit of the digital-to-analog converter, decreasing the amplitude of the compensation voltage signal. Each adjustment requires calculating the change in the digital-to-analog converter input bit corresponding to the deviation of the digital displacement signal. , In the formula, For the first The change in the digital-to-analog converter input code corresponding to the feedback adjustment is expressed in codes. For the first The digital displacement signal before the feedback adjustment is expressed in bits. The center value of the target's middle position interval, in code; This represents the change in digital displacement signal caused by each change of one digital-to-analog converter (DAC) input code, measured in digital displacement signal codes per DAC input code. The new digital input code for the DAC is determined by the following formula: , In the formula, For the first The digital input code used in the secondary feedback adjustment is a digital-to-analog converter, and the unit is code; For the first The digital input code used in the secondary feedback adjustment is a digital-to-analog converter, and the unit is code; This represents the change in the rounded-down numeric input code, expressed in code units.

[0067] If the calculated new digital input code is lower than 0, the new digital input code is restricted to 0. If the calculated new digital input code is higher than the full-scale code of the digital-to-analog converter, the new digital input code is restricted to the full-scale code. After each update of the compensation voltage signal, wait at least 5 times the low-pass filter time constant before reading the new digital displacement signal to avoid over-adjustment caused by the filter not yet stabilizing. When three consecutive position judgment results indicate that the digital displacement signal is within the target's middle position range, the zeroing is confirmed to be complete.

[0068] For example, the center value of the target intermediate position interval is set to 32768 bits. The standard deviation of the digital displacement signal obtained from 200 static measurements is 2 bits, and the half-width of the target intermediate position interval is set to 8 bits, which is four times the standard deviation. Therefore, the target intermediate position interval is 32760 to 32776 bits. The initial digital input code of the digital-to-analog converter is set to 36044 bits, and the corresponding voltage divider control parameters are calculated as follows: The amplitude modulation coefficient corresponding to the reference position is 0.600, the voltage gain of the preamplifier circuit is 20, and the peak value of the AC excitation signal is 1 volt. The DC analog voltage signal is calculated as follows: The corresponding digital displacement signal is calculated as follows: The result, after rounding, is 45875 codes, which is higher than the target midpoint range. After calibration, for every 1-code increase in the digital-to-analog converter input code, the digital displacement signal decreases by 4 codes; the conversion coefficient amplitude of 4 is used during the conversion. The change in digital input code is calculated as follows: The result, after rounding, is code 3277. The updated digital-to-analog converter digital input code is calculated as follows: After the digital input code of the digital-to-analog converter is updated to code 39321, the voltage divider control parameter becomes 0.600, and the digital displacement signal returns to code 32768, which is located in the target midpoint range between code 32760 and code 32776, thus completing the zero-adjustment control.

[0069] S7. Lock the output state of the digital-to-analog converter to fix the compensation voltage signal; obtain the relative displacement change caused by the probe moving to a distance or near, and use the digital displacement signal in the middle position range of the target as a reference to measure the relative displacement change and generate a relative displacement measurement result.

[0070] Specifically, after three consecutive position determinations indicate that the digital displacement signal is within the target's midpoint range, the controller stops generating feedback adjustment commands to change the digital input code of the digital-to-analog converter (DAC). The controller writes the current DAC digital input code into the holding register and prevents ordinary measurement programs from rewriting the holding register. The DAC continues to generate a fixed-amplitude compensation voltage signal based on the locked digital input code and the AC excitation signal. Locking the DAC output state indicates fixed voltage divider control parameters, but does not mean the DAC output is turned off.

[0071] Once locking is complete, the digital displacement signal is continuously sampled and averaged to obtain the zero-adjustment reference digital displacement signal. As the probe moves further or closer, the compensation voltage signal remains constant, while the initial displacement voltage signal varies with the distance between the probe and the target surface. Therefore, the synthesized voltage signal only reflects the relative displacement change.

[0072] The conversion factor between the digital displacement signal and the relative displacement change is determined by a displacement calibration platform. After the digital-to-analog converter output is locked, the displacement calibration platform drives the target surface to move a known distance and records the digital displacement signals before and after the movement. The conversion factor is calculated using the following formula: , In the formula, This is the conversion factor between digital displacement signal and relative displacement change, with the unit being yards per meter; and The digital displacement signal corresponding to the two calibrated positions is expressed in bits. and The known distances between the two calibration positions are in meters. During the actual measurement, the relative displacement measurement results are generated according to the following formula: , In the formula, For the first The relative displacement measurement results corresponding to each measurement are in meters; For the first The digital displacement signal corresponding to each measurement is expressed in bits. This is the digital displacement signal of the zeroing reference recorded after zeroing is completed, in units of codes; This is the conversion factor between digital displacement signal and relative displacement change, with the unit being yards per meter.

[0073] A positive relative displacement measurement result indicates that the probe has moved away from the calibrated direction. A negative relative displacement measurement result indicates that the probe has moved towards the calibrated direction. The directional relationship is fixed during the calibration process and does not change during the measurement. When the digital displacement signal reaches the preset safety boundary on either side of the target's intermediate position range, the controller outputs an out-of-range warning and does not automatically update the compensation voltage signal during the measurement process to avoid changes in the relative displacement measurement reference.

[0074] For example, the digital input code of the digital-to-analog converter is locked at code 39321, and the zero-adjustment reference digital displacement signal is recorded as code 32768. The calibration platform drives the target surface to move 10 micrometers away from the probe, and the digital displacement signal changes from code 32768 to code 34865. The conversion factor is calculated as follows: This translates to 209.7 codes per micrometer. In actual measurement, the digital displacement signal was 34865 codes, and the relative displacement measurement result was calculated as follows: After the probe moves along the approach direction, the digital displacement signal changes to 31090 codes, and the relative displacement measurement result is calculated as follows: Therefore, with the digital-to-analog converter output state locked, using the digital displacement signal within the target's intermediate position range as a reference, it is possible to measure the relative displacement change caused by the probe moving to a distance or closer location, and generate relative displacement measurement results with directional information.

[0075] Reference Figure 4 A specific zeroing process includes the following: In practical use, first install the probe on the device under test, ensuring the target surface is within the probe's effective measurement range and that the probe is parallel to the target surface. The operator then adjusts the probe or target surface to the initial position where relative displacement measurement needs to begin, and then sends a zeroing command to the microcontroller.

[0076] like Figure 5 As shown, after receiving the zero-adjustment command, the microcontroller controls the signal source to output an AC excitation signal. The AC excitation signal, after being buffered, is simultaneously input to the probe and the digital-to-analog converter (DAC). The buffer isolates the probe load from the DAC load, preventing load changes from affecting the amplitude of the AC excitation signal. The probe senses the capacitance formed between the probe and the target surface and modulates the AC excitation signal based on this capacitance to generate an initial displacement voltage signal. Meanwhile, the digital-to-analog converter uses the AC excitation signal as a reference input, scales the same AC excitation signal according to the initial digital input code in an adjustable ratio, and then reverses the phase of the signal through an inverting buffer circuit to generate a compensation voltage signal that is opposite in phase to the initial displacement voltage signal.

[0077] Initial displacement voltage signal and compensation voltage signal The signals are inverted and superimposed at the superposition node by the adder to form a composite voltage signal. The synthesized voltage signal is sequentially amplified into an amplified synthesized signal by a preamplifier circuit. The signal is then subjected to a sinusoidal phase-sensitive detector and low-pass filtering that is in phase and frequency with the AC excitation signal, and superimposed with the input midpoint voltage of the analog-to-digital converter to form a DC analog voltage signal. Finally, the signal is sampled, quantized, and averaged by the analog-to-digital converter to form a digital displacement signal. .

[0078] The microcontroller converts digital displacement signals The position is compared with a preset target midpoint range to generate a position determination result. When the digital displacement signal is higher than the target midpoint range, the microcontroller generates a feedback adjustment command to increase the digital input code of the digital-to-analog converter, increasing the amplitude of the compensation voltage signal and thus increasing the amount of compensation voltage signal canceling the initial displacement voltage signal. When the digital displacement signal is lower than the target midpoint range, the microcontroller generates a feedback adjustment command to decrease the digital input code of the digital-to-analog converter, decreasing the amplitude of the compensation voltage signal and thus decreasing the amount of compensation voltage signal canceling the initial displacement voltage signal. After each update of the compensation voltage signal, the microcontroller waits at least 5 times the low-pass filter time constant before reading the new digital displacement signal and repeats the sampling, judgment, and adjustment process. When three consecutive position determination results indicate that the digital displacement signal is within the target midpoint range, the zeroing is confirmed to be complete.

[0079] After zeroing is completed, the microcontroller writes the current digital input code of the digital-to-analog converter into the holding register and prevents the ordinary measurement program from being rewritten, thus maintaining the compensation voltage signal at a fixed amplitude. At the same time, it continuously samples and averages the digital displacement signal, recording the current digital displacement signal as the zero-position reference. At this point, the compensation voltage signal remains constant, and the initial displacement voltage signal varies with the distance between the probe and the target surface. The synthesized voltage signal only reflects the relative displacement change. If the probe moves away from the target surface, the digital displacement signal changes in one direction; if the probe moves closer to the target surface, the digital displacement signal changes in the opposite direction. The microcontroller uses pre-calibrated conversion factors to determine the digital displacement signal relative to the zero-position reference. Calculation of relative displacement measurement results based on the change value The direction of probe movement is determined by the direction of change. When the digital displacement signal reaches the preset safety boundary on either side of the target's intermediate position range, the microcontroller outputs an out-of-range warning and stops automatically updating the compensation voltage signal during the measurement process to avoid changes in the relative displacement measurement reference.

[0080] Since the compensation voltage signal generated by the digital-to-analog converter originates from the same AC excitation signal, its amplitude and the excitation noise it carries are proportionally scaled from the AC excitation signal. Therefore, it shares the same origin as the base signal component and its excitation noise in the initial displacement voltage signal. When the initial displacement voltage signal and the compensation voltage signal are superimposed in out of phase, the large base signal component corresponding to the initial absolute displacement and its associated excitation noise in the initial displacement voltage signal are canceled out by the bias signal component. The synthesized voltage signal retains only the carrier component corresponding to the relative displacement change and the residual noise that is not completely canceled out, allowing the subsequent circuit to mainly process the smaller signal corresponding to the relative displacement change. Therefore, even if the probe is located at a large range position, it can reduce the signal amplitude and associated noise entering the preamplifier circuit, reduce the possibility of preamplifier circuit saturation, and improve the relative displacement measurement resolution at large displacement positions.

[0081] It should be noted that, without changing the basic principle of this invention, the rectifier and filter circuit can be replaced with a synchronous detector, a lock-in amplifier, or a digital demodulation circuit. When using digital demodulation, the amplified and synthesized signal can first undergo high-speed analog-to-digital conversion, and then the microcontroller or digital signal processor can perform synchronous demodulation based on the phase of the AC excitation signal.

[0082] The digital-to-analog converter can be a voltage output type digital-to-analog converter, a multiplication type digital-to-analog converter, a digital potentiometer, a programmable gain amplifier, or a switched resistor array. As long as the AC excitation signal from the same source can be controllably scaled according to the feedback adjustment command to form a bias signal inversely phase to the initial displacement voltage signal, the bias cancellation function of this invention can be achieved.

[0083] Microcontrollers can also be replaced by digital signal processors, field-programmable gate arrays, or other processors with data acquisition and control functions.

[0084] Based on the same inventive concept, the present invention also provides a capacitive displacement sensor, which employs the zeroing circuit or zeroing method described above, including a target surface and a low-noise zeroing circuit. The target surface is made of a conductive material; The low-noise zeroing circuit includes a signal source, a probe, a digital-to-analog converter, an adder, a preamplifier circuit, a rectifier and filter circuit, an analog-to-digital converter, and a microcontroller. The probe includes a measuring electrode and a shielding electrode that are insulated from each other. A capacitor is formed between the measuring electrode and the target surface. The shielding electrode is used to limit the diffusion of the edge electric field, so that the change in capacitance is mainly caused by the change in distance between the probe and the target surface.

[0085] All equivalent changes and modifications made in accordance with the teachings of this invention are still within the scope of this invention. Those skilled in the art will readily conceive of other embodiments of this invention upon considering the specification and the disclosure of practical truth. This application is intended to cover any variations, uses, or adaptations of this invention that follow the general principles of this invention and include common knowledge or conventional techniques in the art not described herein.

Claims

1. A low-noise zero-adjustment circuit, characterized in that, The circuit includes: A signal source is used to generate AC excitation signals; The probe is connected to the signal source and is used to receive the AC excitation signal and perform modulation operations to generate an initial displacement voltage signal. A digital-to-analog converter, connected to the signal source, is used to acquire the AC excitation signal and perform a digital-to-analog conversion voltage division operation on the AC excitation signal to generate a compensation voltage signal. The compensation voltage signal is out of phase with the initial displacement voltage signal, and the amplitude and excitation noise carried by the compensation voltage signal are scaled proportionally to the AC excitation signal and are from the same source as the excitation noise in the initial displacement voltage signal. The adder, connected to the probe and the digital-to-analog converter respectively, is used to perform inverse superposition processing of the initial displacement voltage signal and the compensation voltage signal to cancel the base signal component corresponding to the absolute displacement and the excitation noise carried in the initial displacement voltage signal, and generate a synthetic voltage signal that retains the relative displacement change. A preamplifier circuit, connected to the adder, is used to amplify the synthesized voltage signal to generate an amplified synthesized signal; A rectifier and filter circuit, connected to the preamplifier circuit, is used to rectify and filter the amplified and synthesized signal to generate a DC analog voltage signal. An analog-to-digital converter, connected to the rectifier and filter circuit, is used to perform analog-to-digital conversion on the DC analog voltage signal to generate a digital displacement signal; The microcontroller is connected to both the analog-to-digital converter and the digital-to-analog converter. It is used to generate a feedback adjustment command using the digital displacement signal and send the feedback adjustment command to the digital-to-analog converter to update the compensation voltage signal, thereby completing the zero-adjustment control.

2. The low-noise zero-adjustment circuit according to claim 1, characterized in that, The digital-to-analog converter (DAC) is a multiplicative DAC, wherein the reference signal input terminal of the multiplicative DAC receives the AC excitation signal, and the digital control terminal receives the voltage divider control parameters output by the microcontroller; or the DAC is combined with an inverting amplifier circuit to convert the in-phase scaling signal output by the DAC into a compensation voltage signal that is out of phase with the initial displacement voltage signal; the DAC includes any one of a voltage output DAC, a digital potentiometer, a programmable gain amplifier, or a switched resistor array.

3. The low-noise zero-adjustment circuit according to claim 1, characterized in that, The rectifier and filter circuit is a phase-sensitive rectifier circuit that is in phase and frequency with the AC excitation signal, or a synchronous detector, lock-in amplifier, or digital demodulation circuit; the microcontroller includes a digital signal processor or a field-programmable gate array.

4. The low-noise zero-adjustment circuit according to claim 1, characterized in that, The microcontroller is used to determine whether the digital displacement signal is within a preset target intermediate position range and generate the feedback adjustment command accordingly. The target intermediate position range is set at the middle position of the input range of the analog-to-digital converter, so that the digital displacement signal is located within the target intermediate position range after zeroing, and a symmetrical effective relative displacement measurement range is maintained on both sides of the target intermediate position range. The microcontroller is also used to lock the output state of the digital-to-analog converter after zeroing to fix the compensation voltage signal.

5. A low-noise zeroing method, employing the zeroing circuit as described in any one of claims 1-4, characterized in that, The method includes: Acquire an AC excitation signal and use the AC excitation signal to perform a modulation operation to generate an initial displacement voltage signal; The AC excitation signal is acquired, and a digital-to-analog conversion voltage divider operation is performed on the AC excitation signal to generate a compensation voltage signal. The compensation voltage signal is out of phase with the initial displacement voltage signal, and the amplitude and excitation noise carried by the compensation voltage signal are scaled proportionally to the AC excitation signal and are from the same source as the excitation noise in the initial displacement voltage signal. The initial displacement voltage signal and the compensation voltage signal are superimposed in opposite phase to cancel the base signal component corresponding to the absolute displacement and the excitation noise carried in the initial displacement voltage signal, thereby generating a synthetic voltage signal that retains the relative displacement change. The synthesized voltage signal is subjected to signal conditioning to generate a DC analog voltage signal; The DC analog voltage signal is converted from analog to digital to generate a digital displacement signal; The digital displacement signal is used to generate a feedback adjustment command, and the compensation voltage signal is updated according to the feedback adjustment command, thereby completing the zero adjustment control.

6. The low-noise zeroing method according to claim 5, characterized in that, The step of acquiring the AC excitation signal and performing a digital-to-analog conversion and voltage division operation on the AC excitation signal to generate a compensation voltage signal includes: The AC excitation signal is connected to the reference input terminal of the digital-to-analog converter, so that the output of the digital-to-analog converter simultaneously contains the effective component of the AC excitation signal and the corresponding homogeneous excitation noise. The voltage division ratio of the digital-to-analog converter is adjusted using the feedback adjustment command or the preset voltage division parameter to generate voltage division control parameters; The AC excitation signal is amplitude scaled using the voltage divider control parameters and then inverted to generate a compensation voltage signal that is out of phase with the initial displacement voltage signal. The amplitude of the compensation voltage signal and the excitation noise it carries are both scaled proportionally to the AC excitation signal.

7. A low-noise zeroing method according to claim 5, characterized in that, The step of superimposing the initial displacement voltage signal and the compensation voltage signal to generate a composite voltage signal includes: Extract the base signal component from the initial displacement voltage signal and the compensation signal component from the compensation voltage signal, wherein the base signal component and the compensation signal component contain excitation noise from the same source; The initial displacement voltage signal and the compensation voltage signal are added in opposite phases, so that the base signal component and the compensation signal component cancel each other out, and the common source excitation noise carried by the base signal component and the compensation signal component cancel each other out, generating a noise-reduced synthetic signal that retains the carrier component corresponding to the relative displacement change and the residual noise as the synthetic voltage signal.

8. A low-noise zeroing method according to claim 5, characterized in that, The step of generating a feedback adjustment command using the digital displacement signal and updating the compensation voltage signal according to the feedback adjustment command includes: Determine whether the digital displacement signal is within a preset target middle position range, and generate a position determination result. The target middle position range is set at the middle position of the input range of the analog-to-digital converter, and a symmetrical relative displacement measurement range is maintained on both sides based on the center value of the target middle position range. Based on the position determination result, a feedback adjustment command is generated; The feedback adjustment command is sent to the digital-to-analog converter to update the amplitude of the compensation voltage signal, so that the digital displacement signal is located in the target middle position range, thereby forming a symmetrical effective relative displacement measurement range on both sides of the zero adjustment position.

9. A low-noise zeroing method according to claim 8, characterized in that, The step of sending the feedback adjustment command to the digital-to-analog converter to update the amplitude of the compensation voltage signal, so that the digital displacement signal is located in the target intermediate position range, further includes: Lock the output state of the digital-to-analog converter to fix the compensation voltage signal; Using the digital displacement signal located in the middle position interval of the target as the zeroing reference, the relative displacement changes generated by the probe moving away from the target surface and moving closer to the target surface are obtained respectively. The relative displacement changes symmetrical on both sides of the middle position interval of the target are measured to generate relative displacement measurement results with direction information.

10. A capacitive displacement sensor, characterized in that, The zeroing method described in any one of claims 5-9 includes a target surface and a low-noise zeroing circuit. The target surface is made of a conductive material; The low-noise zeroing circuit includes a signal source, a probe, a digital-to-analog converter, an adder, a preamplifier circuit, a rectifier and filter circuit, an analog-to-digital converter, and a microcontroller. The probe includes a measuring electrode and a shielding electrode that are insulated from each other. A capacitor is formed between the measuring electrode and the target surface. The shielding electrode is used to limit the diffusion of the edge electric field, so that the change in capacitance is mainly caused by the change in distance between the probe and the target surface.

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