Inductive sensor conditioning circuit based on lc resonant circuit

Abstract

The application provides an inductive sensor conditioning circuit based on LC resonant circuit, which comprises the following steps: an inductive sensor is used as an inductor to form a resonant circuit with a capacitor, an active negative resistance circuit is connected to generate a sinusoidal resonant signal, and a signal processing circuit is used to measure the frequency and amplitude of the resonant signal; since the quality factor of the capacitor in the resonant circuit is much higher than that of the inductive sensor, the frequency and amplitude of the resonant signal can accurately reflect the impedance value of the inductive sensor. The application can accurately measure the impedance change of the inductive sensor through the above method, and realize self-temperature compensation of the sensor by using the measured data and algorithm.

Description

Technical Field

[0001] This invention relates to the field of inductive sensor detection circuit technology, and more specifically, to an inductive sensor conditioning circuit based on an LC resonant circuit. Background Technology

[0002] Inductive sensors are electromechanical conversion devices that convert non-electrical quantities in the environment into changes in the sensor's inductive reactance based on the law of electromagnetic induction. They require signal conditioning circuits to further convert the reactance into directly identifiable signals such as voltage and frequency. Conditioning circuits can be categorized into amplitude conditioning and frequency conditioning based on the type of information they modulate the reactance into. Amplitude conditioning connects the sensor's inductor coil to a bridge or voltage divider circuit, then uses the voltage division relationship of high-frequency signals to convert the sensor's impedance change into a voltage value or other electrical signal. Frequency conditioning, on the other hand, uses the sensor coil as a resonant inductor and forms a resonant circuit with a capacitor; the output frequency of the circuit signal directly reflects the sensor's equivalent inductance.

[0003] The most commonly used voltage amplitude conditioning method is synchronous detection. This method is widely used and rapidly developing, and almost all commercial circuits on the market use this technology. However, synchronous detection is also limited in high-resolution measurements. It requires the extraction of the DC component from the demodulated signal through filters or spectrum analysis after the demodulation unit. If a low-pass filter is used to extract the DC component, the narrow bandwidth of the low-pass filter directly determines the amount of noise remaining after filtering, thus causing a contradiction between the dynamic response and resolution of the measurement system. If digital devices are used for spectrum analysis, although resolution and dynamic response can be guaranteed simultaneously, it also places higher demands on the A / D conversion rate while adding additional high-cost components, further increasing the system cost.

[0004] Patent document CN114440751A (application number: CN202210067606.1) discloses a pulse width detection circuit and an inductive displacement sensor, including an oscillation circuit, a coil resonant circuit, and a modulation circuit. The output terminal of the oscillation circuit is connected to the input terminal of the coil resonant circuit, and the output terminal of the coil resonant circuit is connected to the modulation circuit. The oscillation circuit generates a driving square wave signal and outputs it to the coil resonant circuit. The coil resonant circuit generates a sine wave signal with the same frequency as the driving square wave signal and outputs it to the modulation circuit. The modulation circuit converts the sine wave signal into a pulse width modulation signal whose pulse width is proportional to the measured displacement. However, this patent does not measure the frequency and amplitude of the resonant signal, making it impossible to accurately obtain the impedance change under full-scale conditions. Summary of the Invention

[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide an inductive sensor conditioning circuit based on an LC resonant circuit.

[0006] The inductive sensor conditioning circuit based on an LC resonant circuit provided by the present invention includes an LC resonant circuit, an active negative resistance circuit, and a signal processing circuit.

[0007] The LC resonant circuit is used to select a specific frequency and maintain the frequency stability of the resonant signal.

[0008] The active negative resistance circuit is used to replenish the energy consumed in the LC resonant circuit and maintain signal resonance;

[0009] The LC resonant circuit and the active negative resistance circuit are connected in parallel to generate a resonant signal. The signal processing circuit measures the amplitude and frequency of the resonant signal, and then demodulates the impedance change of the sensor. Based on the relationship between the measured quantity, temperature and sensor impedance, the measured quantity is measured more accurately and adaptively compensated for temperature.

[0010] Preferably, the LC resonant circuit includes an inductive sensor and a capacitor connected in parallel;

[0011] The inductive sensor is used to convert non-electrical quantities in the environment into changes in sensor impedance;

[0012] The capacitor and the inductive sensor form an LC resonant circuit, which is used to assist in adjusting the frequency range of the resonant signal.

[0013] Preferably, the equivalent series inductance of the inductive sensor is L, and the equivalent series resistance of the inductive sensor is R. L The capacitance of the capacitor is C, and the equivalent series resistance of the capacitor is R. C The LC resonant circuit can be equivalently represented by impedance changes as a parallel inductance L P Parallel capacitor C P Parallel resistor R P The parallel configuration has the following values:

[0014]

[0015]

[0016]

[0017] In the formula, Q L The quality factor of the inductor branch is ωL / R. L Q C The quality factor of the capacitor branch is 1 / ωR. C C; ω is the signal angular frequency; at the resonant frequency At the location, parallel inductor L P and parallel capacitor C P The reactances will cancel each other out, and the equivalent output impedance is R. P .

[0018] Preferably, the active negative resistance circuit consists of active devices and resistors, used to compensate for the energy consumed by the resistors in the LC resonant circuit, thereby ensuring that the signal in the LC resonant circuit continues to oscillate. The active negative resistance circuit can be equivalently represented as a negative resistor R. A When the active negative resistance circuit is connected in parallel with the LC resonant circuit and R A ≥R P At that time, R P and R A The resulting parallel resistance is infinitely large, L P and C P This forms an ideal LC resonant circuit, where the impedance in the LC resonant circuit is always 0.

[0019] Preferably, the active negative resistance circuit is constructed using active devices including transistors, MOSFETs, and operational amplifiers in the form of Colpitts circuits and cross-coupled circuits.

[0020] Preferably, the signal processing circuit includes a frequency measurement circuit and an amplitude measurement circuit, which are connected in parallel to measure the frequency information and amplitude information in the resonant signal, respectively.

[0021] Preferably, the frequency measurement circuit includes a hysteresis comparator and a digital frequency meter connected in series. The hysteresis comparator converts the resonant signal into a square wave signal of equal frequency, which is then input to the digital frequency meter to measure the frequency of the square wave signal.

[0022] Preferably, the amplitude measurement circuit includes a precision detector circuit, a low-pass filter, an F / V conversion circuit or an A / D converter connected in series. The precision detector circuit is used to perform full-wave rectification on the resonant signal, the low-pass filter is used to filter the rectified signal into a DC voltage signal, and the F / V conversion circuit or A / D converter is used to convert the DC voltage signal into a digital signal, thereby realizing the measurement of the amplitude of the resonant signal.

[0023] Preferably, when the quality factor of the capacitor is higher than that of the inductive sensor and the LC resonant circuit operates in the current-limiting region, the impedance change of the sensor is reflected by the frequency f and amplitude A of the resonant signal, and the specific relationship is as follows:

[0024]

[0025]

[0026] Among them, I bias This is the bias current set in an active negative resistance circuit.

[0027] Preferably, for inductive sensors, changes in the measured quantity x and temperature T cause variations in the sensor impedances L and R. L The frequency f and amplitude A of the eddy current sensor change monotonically as the impedance of the eddy current sensor changes. Furthermore, equations (4) and (5) show that when the impedance of the eddy current sensor changes, the frequency f and amplitude A of its resonant signal change monotonically. Therefore, the measured frequency f and amplitude A are functions of the measured quantity x and temperature T, expressed as polynomials:

[0028]

[0029]

[0030] The inverse function is expressed as:

[0031]

[0032]

[0033] Where, p ij q ij a ij b ij The linear fitting coefficients are: 1 ≤ i ≤ M; 1 ≤ j ≤ N; M is the number of changes in the measured quantity; N is the number of temperature changes.

[0034] By f i A j Composition of column vectors By a ij and b ij Composition of column vectors and The measurement process is through and Compensation is applied to the measurement results to achieve calibration. and To obtain the optimal estimate, let's assume a total of K measurements are obtained. Then:

[0035]

[0036]

[0037] The optimal estimate of the coefficient vector obtained by the least squares method is:

[0038]

[0039]

[0040] Where S is a vector The matrix formed.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] This invention uses an inductive sensor as an inductor to form an LC resonant circuit with a capacitor, and connects it to an active negative resistance circuit to generate a sinusoidal resonant signal. By measuring the frequency and amplitude of the resonant signal, the impedance change under full-scale conditions can be accurately obtained. This method does not require high-cost components and can achieve high-resolution measurement. Attached Figure Description

[0043] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0044] Figure 1 This is a system overall block diagram of the present invention;

[0045] Figure 2a and Figure 2b These are a resonant circuit and its equivalent model involved in this invention;

[0046] Figure 3 This invention relates to a signal processing circuit. Detailed Implementation

[0047] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0048] Example 1:

[0049] This invention provides an inductive sensor conditioning circuit based on an LC resonant circuit, comprising an LC resonant circuit, an active negative resistance circuit, and a signal processing circuit. The LC resonant circuit is used to select a specific frequency and maintain the frequency stability of the resonant signal. The active negative resistance circuit is used to replenish the energy consumed in the LC resonant circuit and maintain signal resonance. The LC resonant circuit and the active negative resistance circuit are connected in parallel to generate a resonant signal. The signal processing circuit measures the amplitude and frequency of the resonant signal, demodulates the impedance change of the sensor, and then performs more accurate measurement and adaptive temperature compensation based on the relationship between the measured quantity, temperature, and sensor impedance.

[0050] The LC resonant circuit includes an inductive sensor and a capacitor connected in parallel; the inductive sensor is used to convert non-electrical quantities in the environment into changes in sensor impedance; the capacitor and the inductive sensor form an LC resonant circuit to assist in adjusting the frequency range of the resonant signal.

[0051] The equivalent series inductance of the inductive sensor is L, and the equivalent series resistance of the inductive sensor is R. L The capacitance of the capacitor is C, and the equivalent series resistance of the capacitor is R. C The LC resonant circuit can be equivalently represented by impedance changes as a parallel inductance L P Parallel capacitor C P Parallel resistor R P The parallel configuration has the following values:

[0052]

[0053]

[0054]

[0055] In the formula, Q L The quality factor of the inductor branch is ωL / R. L Q C The quality factor of the capacitor branch is 1 / ωR. C C; ω is the signal angular frequency; at the resonant frequency At the location, parallel inductor L P and parallel capacitor C P The reactances will cancel each other out, and the equivalent output impedance is R. P .

[0056] The active negative resistance circuit consists of active components and resistors, used to compensate for the energy consumed by the resistors in the LC resonant circuit, thereby ensuring that the signal in the LC resonant circuit continues to oscillate. The active negative resistance circuit can be equivalently represented as a negative resistor R. A When the active negative resistance circuit is connected in parallel with the LC resonant circuit and R A ≥R P At that time, R P and R A The resulting parallel resistance is infinitely large, L P and C P This forms an ideal LC resonant circuit, where the impedance in the LC resonant circuit is always 0.

[0057] The active negative resistance circuit is constructed using active devices, including transistors, MOSFETs, and operational amplifiers, in the form of Colpitts circuits and cross-coupled circuits.

[0058] The signal processing circuit includes a frequency measurement circuit and an amplitude measurement circuit, which are connected in parallel to measure the frequency and amplitude information of the resonant signal, respectively. The frequency measurement circuit includes a hysteresis comparator and a digital frequency meter connected in series. The hysteresis comparator converts the resonant signal into a square wave signal of equal frequency, which is then input to the digital frequency meter to measure the frequency of the square wave signal. The amplitude measurement circuit includes a precision detector circuit, a low-pass filter, and an F / V conversion circuit or an A / D converter connected in series. The precision detector circuit performs full-wave rectification on the resonant signal, the low-pass filter filters the rectified signal into a DC voltage signal, and the F / V conversion circuit or A / D converter converts the DC voltage signal into a digital signal, thereby achieving the measurement of the amplitude of the resonant signal.

[0059] When the quality factor of the capacitor is higher than that of the inductive sensor and the LC resonant circuit operates in the current-limiting region, the impedance change of the sensor is reflected by the frequency f and amplitude A of the resonant signal. The specific relationship is as follows:

[0060]

[0061]

[0062] Among them, I bias This is the bias current set in an active negative resistance circuit.

[0063] For inductive sensors, changes in the measured quantity x and temperature T cause changes in the sensor impedances L and R. L The frequency f and amplitude A of the eddy current sensor change monotonically as the impedance of the eddy current sensor changes. Furthermore, equations (4) and (5) show that when the impedance of the eddy current sensor changes, the frequency f and amplitude A of its resonant signal change monotonically. Therefore, the measured frequency f and amplitude A are functions of the measured quantity x and temperature T, expressed as polynomials:

[0064]

[0065]

[0066] The inverse function is expressed as:

[0067]

[0068]

[0069] Where, p ij q ij a ij b ij The linear fitting coefficients are: 1 ≤ i ≤ M; 1 ≤ j ≤ N; M is the number of changes in the measured quantity; N is the number of temperature changes.

[0070] By f i A j Composition of column vectors By a ij and b ij Composition of column vectors and The measurement process is through and Compensation is applied to the measurement results to achieve calibration. and To obtain the optimal estimate, let's assume a total of K measurements are obtained. Then:

[0071]

[0072]

[0073] The optimal estimate of the coefficient vector obtained by the least squares method is:

[0074]

[0075]

[0076] Where S is a vector The matrix formed.

[0077] Example 2:

[0078] Example 2 is a preferred example of Example 1.

[0079] This invention proposes a design method for an inductive sensor conditioning circuit based on an LC resonant circuit. The inductive sensor conditioning circuit comprises an LC resonant circuit, an active negative resistance circuit, and a signal processing circuit. It can generate a resonant signal and accurately measure the amplitude A and frequency f of the resonant signal. Both amplitude A and frequency f accurately reflect the equivalent series inductance L and equivalent series resistance R of the inductive sensor. L The changes in x enable accurate measurement and adaptive temperature compensation of the measured quantity x.

[0080] like Figure 1 As shown, an inductive sensor is used as an inductor to form a resonant circuit with a capacitor, and an active negative resistance circuit is connected to generate a sinusoidal resonant signal. The resonant signal is directly input to the frequency measurement circuit to measure the frequency f, and also, after precision detection and VF conversion, is input to the frequency measurement circuit to measure the amplitude A. By measuring the frequency and amplitude of the resonant signal, the impedance change under full-scale conditions can be accurately obtained. This method does not require high-cost components and can achieve high-resolution measurements.

[0081] like Figure 2aThis is a specific design of one type of resonant circuit. The inductive sensor uses a solenoid-type inductive displacement sensor with an equivalent series inductance ranging from 700uH to 1400uH and an equivalent series resistance of 20Ω. The resonant capacitor is selected as 1nF with a quality factor higher than 10000. T1-T4 form a complementary coupling circuit, while T5-T6 and the current-limiting resistor R1 form a current source, providing bias current for the complementary cross-coupling circuit. Together, they form an active negative resistance circuit, which, when connected in parallel with the resonant circuit, can be equivalently modeled as follows: Figure 2b In the form of a negative resistance R A The value is approximately: Among them, g mn For the transconductance of two NMOS transistors, g mp The transconductance of the two PMOS transistors is given.

[0082] like Figure 3 This is a specific design of one type of signal processing circuit. The differential resonant signal generated by the aforementioned resonant circuit is amplified by an instrumentation amplifier. On one hand, it is directly converted into a square wave by a hysteresis comparator and input to a digital frequency meter for frequency measurement. On the other hand, it undergoes encrypted full-wave rectification, low-pass filtering, and VF conversion before being input to the digital frequency meter to obtain a frequency value positively correlated with the amplitude. The VF conversion is used to simplify components and further save costs; it can also be replaced with a higher-precision A / D device. The optocoupler is used for digital-to-analog isolation, contributing to a better signal-to-noise ratio. (Appendix) Figure 3 In this circuit, the instrumentation amplifier uses the AD810 chip, the hysteresis comparator uses the LM311 chip, the full-wave rectifier and low-pass filter uses the OPA2277 chip, the optocoupler isolation uses the 6N137 chip, the VF converter chip is the LM331, and the frequency counter is designed using an FPGA, specifically the CYCLONE IV EP4CE6 series. Measurements show the signal frequency to be 73kHz-122kHz, and the amplitude equivalent frequency to be 21kHz-100kHz.

[0083] Adaptive linear compensation involves fitting the measured frequency f and amplitude A to the measured value x and temperature T. During calibration, the displacement fitting iterations are set to M=2, and the temperature fitting iterations to N=1. Using a temperature chamber and a precision displacement platform, the displacement and temperature are continuously changed, and the displacement x, temperature T, frequency measurement f, and amplitude measurement A are recorded for each change. The optimal estimate of the fitting parameters is then obtained using the least squares method. and The displacement was calculated during the measurement process. The temperature can also be estimated as

[0084] The above examples demonstrate excellent performance, with a measurement nonlinearity error of 0.08%FS and a temperature drift of 681.2ppm / ℃.

[0085] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0086] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A conditioning circuit for an inductive sensor based on an LC resonant circuit, characterized in that, Includes an LC resonant circuit, an active negative resistance circuit, and a signal processing circuit; The LC resonant circuit is used to select a specific frequency and keep the resonant signal frequency stable; The active negative resistance circuit is used to replenish the energy consumed in the LC resonant circuit and maintain signal resonance; The LC resonant circuit generates a resonant signal after being connected in parallel with the active negative resistance circuit. The signal processing circuit measures the amplitude and frequency of the resonant signal, and then demodulates the impedance change of the sensor. Based on the relationship between the measured quantity, temperature and sensor impedance, the measured quantity is measured more accurately and adaptively compensated for temperature. The signal processing circuit includes a frequency measurement circuit and an amplitude measurement circuit, which are connected in parallel to measure the frequency and amplitude information in the resonant signal, respectively. The amplitude measurement circuit includes a precision detector circuit, a low-pass filter, an F / V conversion circuit or an A / D converter connected in series. The precision detector circuit is used to perform full-wave rectification on the resonant signal. The low-pass filter is used to filter the rectified signal into a DC voltage signal. The F / V conversion circuit or A / D converter is used to convert the DC voltage signal into a digital signal, thereby realizing the measurement of the amplitude of the resonant signal.

2. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 1, characterized in that, The LC resonant circuit includes an inductive sensor and a capacitor connected in parallel; The inductive sensor is used to convert non-electrical quantities in the environment into changes in sensor impedance; The capacitor and the inductive sensor form an LC resonant circuit, which is used to assist in adjusting the frequency range of the resonant signal.

3. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 2, characterized in that, The equivalent series inductance of the inductive sensor is The equivalent series resistance of the inductive sensor is The capacitance of the capacitor is The equivalent series resistance of the capacitor is The LC resonant circuit can be equivalently represented by impedance changes as a parallel inductance circuit. Parallel capacitors Parallel resistors The parallel configuration has the following values: In the formula, The quality factor of the inductor branch is [value missing]. ; Let be the quality factor of the capacitor branch, and its value is . ; The signal angular frequency; at the resonant frequency Parallel inductor and parallel capacitors The reactances will cancel each other out, at which point the equivalent output impedance is .

4. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 3, characterized in that, The active negative resistance circuit consists of active components and resistors, used to compensate for the energy consumed by the resistors in the LC resonant circuit, thereby ensuring that the signal in the LC resonant circuit continues to oscillate. The active negative resistance circuit can be equivalently represented as a negative resistor. When the active negative resistance circuit is connected in parallel with the LC resonant circuit and hour, and The resulting parallel resistance is infinite. and This forms an ideal LC resonant circuit, where the impedance in the LC resonant circuit is always 0.

5. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 4, characterized in that, The active negative resistance circuit is constructed using active devices, including transistors, MOSFETs, and operational amplifiers, in the form of Colpitts circuits and cross-coupled circuits.

6. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 1, characterized in that, The frequency measurement circuit includes a hysteresis comparator and a digital frequency meter connected in series. The hysteresis comparator converts the resonant signal into a square wave signal of equal frequency, which is then input to the digital frequency meter to measure the frequency of the square wave signal.

7. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 3, characterized in that, When the quality factor of the capacitor is higher than that of the inductive sensor and the LC resonant circuit operates in the current-limited region, the frequency of the resonant signal is... and amplitude The information reflects the impedance change of the sensor, and the specific relationship is as follows: in, This is the bias current set in an active negative resistance circuit.

8. The inductive sensor conditioning circuit based on an LC resonant circuit according to claim 7, characterized in that, For inductive sensors, the measured quantity and temperature Changes cause sensor impedance and The frequency of the resonant signal changes and is monotonic. Furthermore, as shown in equations (4) and (5), when the impedance of the inductive sensor changes, the frequency of its resonant signal changes. and amplitude Consequently, the frequency changes monotonically, therefore the measured frequency and amplitude It is about the measured and temperature The function, expressed as a polynomial, is: The inverse function is expressed as: in, , , , These are the linear fitting coefficients; 1 ≤ ≤M;1≤ ≤N; M is the number of changes in the measured quantity; N is the number of temperature changes; Depend on Composition of column vectors ,Depend on and Composition of column vectors and The measurement process is conducted through and Compensation is applied to the measurement results to achieve calibration. and To obtain the optimal estimate, let's assume a total of K measurements are obtained. Then: The optimal estimate of the coefficient vector obtained by the least squares method is: in, For vector , ,…, The matrix formed.

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