Low-temperature drift digital phase-locked eddy current sensor circuit

Abstract

The invention, which relates to the technical field of the precision measuring instrument, discloses a low-temperature-drift digital phase-locked eddy current sensor circuit comprising an MCU, an excitation signal generation module, an AC bridge module, and a high-speed precision ADC sampling module. The excitation signal generation module generates a high-stability 50kHz sine wave excitation signal through high-precision DAC and DMA transmission by adopting a DDS (Direct Digital Synthesizer) mode; the AC bridge module suppresses common-mode interference caused by probe temperature excursion by using a reference coil; the high-speed precise ADC sampling module is used for synchronously digitizing bridge output and excitation signals; the temperature compensation module performs real-time software compensation on the residual temperature excursion through an integrated digital temperature sensor and a pre-calibrated polynomial model; a digital phase-locked demodulation algorithm is realized in the MCU, and a displacement related amplitude is accurately extracted through digital multiplication and low-pass filtering. The circuit fundamentally inhibits temperature drift from hardware and software levels, and realizes high-precision and high-stability eddy current displacement measurement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of precision measuring instruments, in particular to a low-temperature digital phase-locked eddy current sensor circuit. BACKGROUND

[0002] Eddy current sensors are widely used in the precise measurement of displacement, vibration, thickness and other parameters in the fields of industrial online detection, aerospace, precision machining and scientific experiments due to their non-contact, high resolution, high response frequency and other advantages. Traditional analog eddy current sensors usually adopt LC oscillator or analog phase-locked loop (APLL) structure, and the measurement accuracy of the sensors is easily affected by environmental temperature changes. The main sources of temperature drift include: 1) temperature drift of analog components, among which the temperature drift of passive components can be effectively suppressed by selecting low-temperature drift components, and the most difficult to suppress is the temperature drift of operational amplifiers, analog phase-locked loops and other integrated circuit chips; 2) temperature drift of probe coils, the resistance and inductance of the sensor probe coils change with temperature, directly changing the sensitivity and zero point of the sensor; 3) the distributed capacitance and inductance of the probe connector and cable are also affected by temperature. These factors jointly cause slow changes in the output signal of the sensor and produce measurement errors, which seriously restrict the application of the sensor in ultra-high precision and long-term stability requirements.

[0003] To solve the problem of temperature drift, the existing technology mainly adopts hardware compensation technology, such as using low-temperature drift components, designing symmetric differential circuits or adding additional temperature sensors for software compensation. These methods have the disadvantages of high cost, complex process, limited compensation effect and poor universality. SUMMARY

[0004] The purpose of the present application is to overcome the technical defects of existing analog phase-locked eddy current sensors, such as serious temperature drift, poor long-term stability and complex compensation circuit, and to provide a digital solution that fundamentally suppresses temperature drift and improves measurement accuracy.

[0005] The technical solution of the present application is to provide a low-temperature digital phase-locked eddy current sensor circuit, which comprises: an MCU, an excitation signal generation module, an AC bridge module and a high-speed precision ADC sampling module.

[0006] The MCU uses an external temperature-compensated crystal oscillator as a clock source, and the MCU is electrically connected with each module to realize data processing through direct memory access transmission, for controlling the generation of excitation signals, signal sampling and digital phase-locked demodulation.

[0007] The excitation signal generation module includes a DAC, a signal conditioning circuit, and a power amplifier. The MCU writes data into the DAC, which generates a 50kHz sine wave excitation signal using direct digital synthesis. The DAC is followed by a series in-phase amplifier circuit and a series DC blocking capacitor to filter out the DC component. Finally, a series power amplifier is connected to improve the load capacity.

[0008] The AC bridge module adopts an AC bridge structure with a reference coil. The AC bridge module includes a probe coil, an AC bridge, a reference coil, and a follower. The two adjacent bridge arms are the measuring coil and the reference coil, respectively. The reference coil is wound with a low-temperature drift coefficient material. In the balanced position, the bridge is adjusted to a balanced state by adjusting the resistance and the inductance of the reference coil. When the measuring coil approaches or moves away from the target conductor, a displacement-related differential voltage signal is generated. A differential preamplifier is connected in series after the bridge to adjust the signal to the ADC sampling range.

[0009] The high-speed precision ADC sampling module includes a differential amplifier and an analog-to-digital converter (ADC). A preamplifier and a programmable gain amplifier are set before the ADC to amplify and condition the bridge output signal so that the amplitude matches the ADC input range. The ADC synchronously samples and digitizes the conditioned bridge output voltage and the excitation signal.

[0010] A digital phase-locked demodulation algorithm is implemented in an MCU. The excitation signal is multiplied by the bridge output signal using a multiplier, and the amplitude is extracted by filtering out high-frequency components using a digital low-pass filter. The bridge output signal V... in and excitation signal V ref The expression is:

[0011] ;

[0012] ;

[0013] Where Re represents the real part, Im represents the imaginary part, U is the bridge output voltage, ω is the angular frequency of the excitation signal, and the phase θ of the excitation signal is adjustable;

[0014] When the phase θ of the excitation signal is 0°, the signal after multiplication by the multiplier is filtered by a digital low-pass filter to obtain the DC output V. out :

[0015] ;

[0016] It can accurately extract amplitude and phase information related to displacement, and achieve accurate and rapid tracking and demodulation of probe impedance changes.

[0017] In any of the above technical solutions, further, the multiplier will convert the bridge output signal V... in and excitation signal V ref Multiplying them together gives:

[0018] ;

[0019] When the excitation signal phase θ = 0°, based on the identity of the two signal multiplication is:

[0020] .

[0021] In any of the above technical solutions, further, the output voltage amplitude expression of the AC bridge is:

[0022] ;

[0023] Wherein, e is the amplitude of the power amplifier output AC voltage, R s is the bridge arm resistance of the AC bridge, ΔR is the resistance change caused by the detection coil close to or away from the measured conductor, ΔL is the inductance change, R is the reference coil resistance, L is the reference coil inductance, j is the imaginary unit in electrical engineering.

[0024] In any of the above technical solutions, further, the circuit further comprises a temperature compensation module;

[0025] The temperature compensation module integrates a digital temperature sensor on the printed circuit board near the position of the key analog components to monitor the temperature, and through the pre-calibration test, a polynomial mathematical model of the sensor zero point output and the sensitivity change with temperature is established, the model and the temperature value are written into the processor on-chip flash memory, and the temperature is inquired before the output displacement value and the original amplitude calculated by the digital phase-locked loop is compensated in real time.

[0026] In any of the above technical solutions, further, the DAC is a 16-bit voltage output high-speed digital-to-analog converter with 2.5V, 2ppm / ℃ internal reference voltage.

[0027] In any of the above technical solutions, further, the ADC is a 16-bit, 1MSPS conversion rate analog-to-digital converter.

[0028] In any of the technical solutions above, further, the working process of the circuit comprises: the MCU controls the excitation signal generation module to generate a 50 kHz sine wave excitation signal, which drives the detection coil and the reference coil after being converted by the DAC, adjusted by the signal conditioning circuit, and amplified by the power amplifier; the detection coil detects the impedance change caused by displacement, generates a differential voltage signal through the AC bridge, and outputs to the high-speed precision ADC sampling module after being adjusted by the differential preamplifier; the differential amplifier amplifies and conditions the signal, and the ADC performs 16-bit, 1MSPS synchronous sampling to convert the digital signal to the MCU; digital phase-locked demodulation is realized in the MCU, the excitation signal is multiplied with the bridge output signal through the multiplier, high-frequency components are filtered out through the low-pass filter to extract the amplitude; the temperature sensor monitors the temperature, the MCU compensates the original amplitude according to the polynomial model, and outputs the displacement measurement value.

[0029] The beneficial effects of the present application are:

[0030] The present application replaces the traditional analog phase-locked loop with a fully digital phase-locked loop structure, combines the AC bridge with a reference coil, a high-stability DDS excitation source, synchronous high-speed ADC sampling, and multiple measures of software temperature compensation, effectively eliminates the main temperature drift sources of the operational amplifier, the probe coil, and the cable, significantly reduces the sensitivity of the sensor output to environmental temperature changes, improves the long-term measurement stability and reliability, and reduces the calibration and maintenance frequency; at the same time, higher measurement accuracy, resolution, and response speed are realized, the flexibility of system parameter adjustment, the non-linear correction capability, and the intelligent level are enhanced, which is convenient for integration into modern measurement and control systems and is suitable for industrial detection occasions with high precision and long-term stability requirements. BRIEF DESCRIPTION OF DRAWINGS

[0031] The advantages of the above and additional aspects of the present application will become apparent and easily understood in connection with the following description of embodiments, taken in conjunction with the following drawings, in which:

[0032] Figure 1 is a structural schematic diagram of a low-temperature digital phase-locked eddy current sensor circuit according to an embodiment of the present application;

[0033] Figure 2 is a principle block diagram of a digital phase-locked loop of a low-temperature digital phase-locked eddy current sensor circuit according to an embodiment of the present application;

[0034] Figure 3 is a curve diagram of the output voltage change of the lock-in amplifier, which is obtained by adding a ±10% deviation to the inductance parameter of the detection coil of a low-temperature digital phase-locked eddy current sensor circuit according to an embodiment of the present application on the basis of the reference value, and selecting 10 points for parameter scanning. DETAILED DESCRIPTION

[0035] In order to more clearly understand the above-mentioned purposes, features and advantages of the present application, the present application will be further described in detail below in conjunction with the drawings and specific embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0036] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, however, the present application can also be practiced in other ways different from those described herein, and therefore, the scope of protection of the present application is not limited by the specific embodiments disclosed below.

[0037] With the development of digital signal processing (DSP) technology and field programmable gate array (FPGA), digital phase-locked technology realizes phase, frequency tracking and measurement of signals in a digital way, fundamentally solves the problem of obvious temperature drift of analog phase-locked loop circuit, and has much better stability than analog circuit.

[0038] As shown in Figure 1 The embodiment provides a low-temperature digital phase-locked eddy current sensor circuit, which replaces a traditional analog phase-locked structure by a full-digital signal processing link, cooperatively suppresses measurement drift caused by environmental temperature from a physical layer and an algorithm layer, and the circuit comprises an MCU, an excitation signal generation module, an alternating current bridge module, a high-speed precision ADC sampling module and a temperature compensation module.

[0039] The excitation signal generation module comprises a DAC, a signal conditioning circuit and a power amplifier; the alternating current bridge module comprises a detection coil, an alternating current bridge, a reference coil and a follower; the high-speed precision ADC sampling module comprises a differential amplifier and an ADC; and the temperature compensation module comprises a temperature sensor.

[0040] The excitation signal generation module is used for generating a high-stability and low-noise sinusoidal excitation signal; the alternating current bridge module is used for suppressing common-mode interference caused by temperature drift of a probe; the high-speed precision ADC sampling module is used for converting an analog signal into a digital signal; and the temperature compensation module is used for correcting residual system-level temperature drift. Through the organic combination of the modules, the present application fundamentally suppresses measurement drift caused by environmental temperature change, and improves the measurement precision and stability.

[0041] The MCU is used as a control core of the system, selects a high-main-frequency processor, and is equipped with an external high-precision and low-temperature drift temperature compensation crystal oscillator (TCXO) to ensure the high-frequency stability of a system clock. The MCU is electrically connected with each module, and is used for coordinating generation of an excitation signal, sampling processing of a signal and execution of temperature compensation.

[0042] The excitation signal generation module includes a DAC, a signal conditioning circuit and a power amplifier. The module generates the excitation signal in a direct digital synthesis (DDS) mode: the number of sampling points of a sine wave in a period is calculated in advance in the memory of an MCU, a high-precision timer is used to trigger direct memory access (DMA) transmission at an accurate frequency, and the waveform data is written into the DAC without direct intervention of the CPU, thereby avoiding frequency and amplitude jitter caused by interrupt delay and task scheduling and ensuring that the stability of the excitation signal is extremely high.

[0043] The DAC is selected to be a 16-bit voltage output high-speed digital-to-analog converter with an internal reference voltage of 2.5 V and 2 ppm / °C, which has a settling time of ±0.003% FSR in 10 μs, low noise and low temperature drift; a same-phase proportional amplification circuit composed of a low-noise, low-distortion and high-voltage slew rate operational amplifier is connected in series at the rear stage of the DAC, which is used to adjust the amplitude of the signal and filter out the DC component in series with a DC blocking capacitor, and finally a high-performance power amplifier is connected in series to improve the load capacity.

[0044] The sine wave frequency generated by the excitation signal generation module is 50 kHz, which is used to drive the eddy current sensor probe and ensure the stability of the amplitude and frequency of the excitation source relied on impedance measurement.

[0045] The AC bridge module includes a detection coil, an AC bridge, a reference coil and a follower. The module adopts an AC bridge structure with a reference coil, which is the key to suppressing common-mode interference caused by probe temperature drift. Among them, two adjacent bridge arms are a measurement coil and a reference coil respectively, the measurement coil is used to detect the distance between the target conductor, and the reference coil is wound by a low-temperature drift coefficient material; at the balance position, the bridge is adjusted to the balance state by adjusting the resistance and the inductance of the reference coil; when the measurement coil approaches or moves away from the target conductor, the bridge balance is broken, a displacement-related differential voltage signal is generated, and thus the displacement is measured. The output voltage amplitude expression of the AC bridge is:

[0046] ;

[0047] wherein U is the output voltage of the bridge, e is the amplitude of the output AC voltage of the power amplifier, R s is the resistance of the bridge arm of the AC bridge, ΔR is the resistance change amount caused by the detection coil approaching or moving away from the measured conductor, ΔL is the inductance change amount, R is the resistance of the reference coil, L is the inductance of the reference coil, j is the imaginary unit in electrical engineering, and ω is the angular frequency of the excitation signal; it can be seen from the formula that the inductance change or the resistance change caused by the displacement change can cause the change of the output voltage amplitude of the bridge; in order to improve the sensitivity, a differential preamplifier is connected in series at the rear stage of the bridge to adjust the signal to the range suitable for ADC sampling.

[0048] The high-speed precision ADC sampling module comprises a differential amplifier and an ADC. The module sets a preamplifier and a programmable gain amplifier composed of a low-noise, low-temperature drift operational amplifier before the ADC, preliminarily amplifies and conditions the weak signal output by the bridge, and matches the amplitude of the signal to the optimal input range of the ADC, so as to fully utilize the dynamic range of the ADC; the ADC with 16 bits and a conversion rate of 1 MSPS is used to digitize the bridge output voltage and the excitation signal output by the power amplifier after synchronous sampling; the sampling process ensures high-quality conversion of the signal, and provides reliable digital signals for subsequent digital signal processing.

[0049] The temperature compensation module comprises a temperature sensor. A digital temperature sensor is integrated on a PCB close to key analog components (such as a drive amplifier and an ADC reference source) to monitor the temperature of key components on the circuit board; through preliminary calibration tests, a polynomial mathematical model of the zero-point output and sensitivity of the sensor changing with temperature is established; the model and the temperature value read by the temperature sensor in real time are written into the on-chip Flash of the processor; before the final output of the displacement value, the processor queries the current temperature and the established temperature drift model to compensate and correct the original amplitude calculated by the digital phase-locked loop in real time, and finally outputs the high-precision displacement measurement value after temperature compensation; the compensation algorithm enhances the flexibility and intelligent level of the system, facilitates adjustment of sensor parameters such as range, bandwidth, and sensitivity, and realizes nonlinear correction, multi-point temperature compensation, self-diagnosis, and digital communication, and is more easily integrated into modern measurement and control systems.

[0050] The working process of the circuit is as follows: first, the MCU controls the excitation signal generation module to generate a 50 kHz sinusoidal excitation signal, which is converted by the DAC, adjusted in amplitude and filtered for DC components by the signal conditioning circuit, and then amplified by the power amplifier to drive the detection coil and the reference coil in the AC bridge module; the detection coil detects the impedance change caused by the displacement of the target conductor, and the reference coil provides a reference, and a differential voltage signal is generated through the AC bridge, which is buffered by the follower and then output to the high-speed precision ADC sampling module; the differential amplifier in the high-speed precision ADC sampling module amplifies and conditions the signal to match the ADC input range, and then the ADC performs 16-bit, 1 MSPS synchronous sampling to convert the analog signal into a digital signal and transmit it to the MCU.

[0051] The digital phase-locked demodulation algorithm is realized in the MCU, such as Figure 2As shown, synchronous demodulation is achieved through a digital lock-in amplifier (LIA), which includes a multiplier (MIXER) and a low-pass filter (LPF). In the digital domain, the excitation signal is multiplied by the bridge output signal, and then the signal is filtered out by the digital low-pass filter to extract the amplitude, thereby minimizing signal interference from other frequency components and reducing noise in the signal amplitude measurement. The extracted amplitude includes information about the displacement between the measured conductor and the probe. The bridge output signal V... in and excitation signal V ref The expression is:

[0052] ;

[0053] ;

[0054] Where Re represents the real part, Im represents the imaginary part, and the phase θ of the excitation signal is adjustable.

[0055] The multiplier will output the bridge signal V in and excitation signal V ref Multiplying them together, we get:

[0056] ;

[0057] Based on trigonometric identities:

[0058] ;

[0059] When the excitation signal phase θ = 0°:

[0060] ;

[0061] The above signal, after passing through a low-pass filter, yields a DC output V. out :

[0062] ;

[0063] The real part of the signal can be obtained by using an excitation signal with a 0° phase angle. By amplifying the signal through this lock-in amplifier, interference can be eliminated to the maximum extent and noise can be reduced.

[0064] like Figure 3 As shown, based on the formula calculation, the inductance parameter of the detection coil was adjusted by ±10% to the reference value. Ten points were selected for parameter scanning, resulting in ten sets of output voltage variation curves of the lock-in amplifier. It can be seen from the figure that when the inductance value changes by ±10%, the output voltage amplitude of the lock-in amplifier varies between 0.74V and -0.87V, and is proportional to the change in inductance. This demodulation process achieves accurate and rapid tracking and demodulation of the probe impedance change, thereby obtaining high measurement resolution and accuracy.

[0065] Subsequently, the temperature sensor in the temperature compensation module monitors the temperature of the key position in real time, and the MCU compensates and corrects the demodulated original amplitude in real time according to the pre-stored polynomial mathematical model, and finally outputs a high-precision displacement measurement value.

[0066] In summary, the application provides a low-temperature digital phase-locked eddy current sensor circuit, which comprises an MCU, an excitation signal generation module, an alternating current bridge module and a high-speed precision ADC sampling module.

[0067] The MCU uses an external temperature compensation crystal oscillator as a clock source, and is electrically connected with each module to realize data processing through direct memory access transmission, and is used for controlling the generation of excitation signals, signal sampling, digital phase-locked demodulation and temperature compensation.

[0068] The excitation signal generation module comprises a DAC, a signal conditioning circuit and a power amplifier; a 50kHz sine wave excitation signal is generated in a direct digital synthesis mode, the number of sine wave sampling points is pre-calculated in the MCU memory, data is written into the DAC through high-precision timer triggering direct memory access transmission, a same-phase proportional amplification circuit composed of a low-noise, low-distortion and high-pressure swing rate operational amplifier is connected in series at the rear stage of the DAC, a direct current component is filtered out through a direct current blocking capacitor connected in series, and finally a power amplifier is connected in series to improve the load capacity.

[0069] The alternating current bridge module adopts an alternating current bridge structure with a reference coil, and comprises a detection coil, an alternating current bridge, a reference coil and a follower; two adjacent bridge arms are respectively a measurement coil and a reference coil; the reference coil is wound by a low-temperature drift coefficient material; in the balanced position, the bridge is adjusted to a balanced state through adjusting resistance and reference coil inductance; when the measurement coil approaches or moves away from the target conductor, a differential voltage signal related to displacement is generated; a differential preamplifier is connected in series at the rear stage of the bridge to adjust the signal to the ADC sampling range.

[0070] The high-speed precision ADC sampling module comprises a differential amplifier and an analog-to-digital converter ADC; a preamplifier composed of a low-noise, low-temperature drift operational amplifier and a programmable gain amplifier are arranged before the ADC; the bridge output signal is amplified and conditioned to match the amplitude with the ADC input range; the ADC synchronously samples and digitizes the conditioned bridge output voltage and the excitation signal.

[0071] The digital phase-locked demodulation algorithm is realized in the MCU; the excitation signal and the bridge output signal are multiplied through a multiplier; high-frequency components are filtered out through a digital low-pass filter to extract the amplitude; the expression of the bridge output signal V in and the excitation signal V ref is as follows:

[0072] ;

[0073] ;

[0074] Wherein, Re represents real part, Im represents imaginary part, U is bridge output voltage, and omega is angular frequency of excitation signal.

[0075] When the phase of excitation signal is 0°, the signal multiplied by the multiplier after digital low-pass filter obtains direct current output V out :

[0076] ;

[0077] The amplitude and phase information related to displacement are accurately extracted, and the impedance change of the probe is accurately and rapidly tracked and demodulated.

[0078] The steps in the application can be adjusted in sequence, combined and deleted according to actual needs.

[0079] The units in the device can be combined, divided and deleted according to actual needs.

[0080] In the application, the terms such as "mounting", "connecting", "connecting", "fixing" and the like should be understood broadly, for example, "connecting" can be fixed connection, or detachable connection, or integrally connected; "connecting" can be directly connected, or indirectly connected through an intermediate medium. For those skilled in the art, the specific meaning of the above terms in the application can be understood according to the specific circumstances.

[0081] The shape of each component in the drawings is schematic, and there may be some difference with the real shape, the drawings are only used to illustrate the principle of the application, and are not intended to limit the application.

[0082] Although the application is disclosed in detail with reference to the drawings, it should be understood that the description is only exemplary, and is not intended to limit the application of the application. The protection scope of the application is defined by the appended claims, and can include various modifications, modifications and equivalent schemes made to the application without departing from the protection scope and spirit of the application.

Claims

1. A low-temperature drift digital phase-locked loop eddy current sensor circuit, characterized in that, The circuit includes: an MCU, an excitation signal generation module, an AC bridge module, and a high-speed precision ADC sampling module; The MCU uses an external temperature-compensated crystal oscillator as its clock source. The MCU is electrically connected to each module and performs data processing through direct memory access. It is used to control the generation of excitation signals, signal sampling, and digital phase-locked demodulation. The excitation signal generation module includes a DAC, a signal conditioning circuit, and a power amplifier. The MCU writes data into the DAC, which generates a 50kHz sine wave excitation signal using direct digital synthesis. The DAC is followed by a series in-phase amplifier circuit and a series DC blocking capacitor to filter out the DC component. Finally, a series power amplifier is connected to improve the load capacity. The AC bridge module adopts an AC bridge structure with a reference coil. The AC bridge module includes a probe coil, an AC bridge, a reference coil, and a follower. The two adjacent bridge arms are the measuring coil and the reference coil, respectively. The reference coil is wound with a low-temperature drift coefficient material. In the balanced position, the bridge is adjusted to a balanced state by adjusting the resistance and the inductance of the reference coil. When the measuring coil approaches or moves away from the target conductor, a displacement-related differential voltage signal is generated. A differential preamplifier is connected in series after the bridge to adjust the signal to the ADC sampling range. The high-speed precision ADC sampling module includes a differential amplifier and an analog-to-digital converter (ADC). A preamplifier and a programmable gain amplifier are set before the ADC to amplify and condition the bridge output signal so that the amplitude matches the ADC input range. The ADC synchronously samples and digitizes the conditioned bridge output voltage and the excitation signal. A digital phase-locked demodulation algorithm is implemented in an MCU. The excitation signal is multiplied by the bridge output signal using a multiplier, and the amplitude is extracted by filtering out high-frequency components using a digital low-pass filter. The bridge output signal V... in and excitation signal V ref The expression is: ； ； Where Re represents the real part, Im represents the imaginary part, U is the bridge output voltage, ω is the angular frequency of the excitation signal, and the phase θ of the excitation signal is adjustable; When the phase θ of the excitation signal is 0°, the signal after multiplication by the multiplier is filtered by a digital low-pass filter to obtain the DC output V. out : ； It can accurately extract amplitude and phase information related to displacement, and achieve accurate and rapid tracking and demodulation of probe impedance changes.

2. The low-temperature drift digital phase-locked loop eddy current sensor circuit as described in claim 1, characterized in that, The multiplier will output signal V from the bridge. in and excitation signal V ref Multiplying them together gives: ； When the phase θ of the excitation signal is 0°, the product of the two signals based on the trigonometric identity is: 。 3. The low-temperature drift digital phase-locked loop eddy current sensor circuit as described in claim 1, characterized in that, The expression for the output voltage amplitude of the AC bridge is: ； Where e is the amplitude of the AC output voltage of the power amplifier, and R s ΔR is the resistance of the bridge arm of the AC bridge, ΔR is the change in resistance caused by the probe coil moving closer to or further away from the conductor being measured, ΔL is the change in inductance, R is the resistance of the reference coil, L is the inductance of the reference coil, and j is the imaginary unit in electrical engineering.

4. The low-temperature drift digital phase-locked loop eddy current sensor circuit as described in claim 1, characterized in that, The circuit also includes a temperature compensation module; The temperature compensation module integrates a digital temperature sensor on the printed circuit board near the key analog components to monitor the temperature. Through preliminary calibration experiments, a polynomial mathematical model of the sensor's zero-point output and sensitivity as a function of temperature is established. The model and temperature values ​​are written into the processor's on-chip flash memory. The temperature is queried before the output displacement value, and the original amplitude calculated by the digital phase-locked loop is compensated in real time.

5. The low-temperature drift digital phase-locked loop eddy current sensor circuit as described in claim 1, characterized in that, The DAC is a 16-bit voltage-output high-speed digital-to-analog converter with an internal reference voltage of 2.5V and 2ppm / ℃.

6. The low-temperature drift digital phase-locked loop eddy current sensor circuit as described in claim 1, characterized in that, The ADC is a 16-bit analog-to-digital converter with a conversion rate of 1 MSPS.

7. The low-temperature drift digital phase-locked loop eddy current sensor circuit as described in claim 4, characterized in that, The circuit's workflow includes: the MCU controls the excitation signal generation module to generate a 50kHz sine wave excitation signal, which is then converted by a DAC, adjusted by a signal conditioning circuit, and amplified by a power amplifier to drive the detection coil and reference coil; the detection coil detects the impedance change caused by displacement, generates a differential voltage signal through an AC bridge, and outputs it to the high-speed precision ADC sampling module after adjustment by a differential preamplifier; the differential amplifier amplifies and conditions the signal, and the ADC performs 16-bit, 1MSPS synchronous sampling to convert it into a digital signal for transmission to the MCU; digital phase-locked demodulation is implemented in the MCU, multiplying the excitation signal with the bridge output signal through a multiplier, filtering out high-frequency components through a low-pass filter to extract the amplitude; a temperature sensor monitors the temperature, and the MCU compensates for the original amplitude according to a polynomial model, outputting the displacement measurement value.

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