阅读说明：本技术 提高交流电桥测量电路测量精度的方法 (Method for improving measurement precision of alternating current bridge measurement circuit ) 是由 高尚华 薛兵 于 2020-12-01 设计创作，主要内容包括：本发明公开了一种提高交流电桥测量电路测量精度的方法,即在原有串联的交流测量电桥、放大电路和相敏检波电路中增设增量-总和调制电路和数字抽取滤波器；交流测量电桥、放大电路、相敏检波电路和所述增量-总和调制电路彼此串联构成负反馈闭环回路。本发明通过在测量电路中增设增量-总和调制电路,采用较高的放大电路增益和交流测量电桥正弦交流激励信号的幅值,抑制相敏检波电路的解调误差和解调噪声,抑制信号通带内的量化噪声。本发明测量精度高,可广泛应用于差动式电容位移传感器、差动变压器和测温仪中。(The invention discloses a method for improving the measurement accuracy of an alternating current bridge measurement circuit, namely, an increment-sum modulation circuit and a digital extraction filter are additionally arranged in an original alternating current measurement bridge, an amplification circuit and a phase sensitive detection circuit which are connected in series; the alternating current measuring bridge, the amplifying circuit, the phase-sensitive detection circuit and the increment-sum modulation circuit are connected in series to form a negative feedback closed loop. The invention adopts higher amplification circuit gain and amplitude of AC measurement bridge sine AC excitation signal to inhibit demodulation error and demodulation noise of phase sensitive detection circuit and inhibit quantization noise in signal pass band by adding increment-sum modulation circuit in measurement circuit. The invention has high measurement precision and can be widely applied to differential capacitance displacement sensors, differential transformers and temperature measuring instruments.)

1. A method for improving the measurement accuracy of an alternating current bridge measurement circuit is characterized in that: an increment-sum modulation circuit is additionally arranged in an original serially connected alternating current measuring bridge, an amplifying circuit and a phase sensitive detection circuit; the alternating current measuring bridge, the amplifying circuit, the phase-sensitive detection circuit and the increment-sum adjusting circuit are connected in series to form a negative feedback closed loop;

the increment-sum modulation circuit is composed of a feedforward integration circuit, an A/D conversion circuit and a D/A conversion circuit which are connected in series;

the signal output by the A/D conversion circuit in the increment-sum modulation circuit is divided into two paths, and one path outputs a digital signal; the other path of the signal is connected with a signal input end of a D/A conversion circuit in the increment-sum adjusting circuit, a feedback signal is generated by the D/A conversion circuit and is fed back to a measuring signal input end of the alternating current measuring bridge or the input end of the amplifying circuit;

by adding an increment-sum modulation circuit in the measuring circuit and adopting higher gain of the amplifying circuit and amplitude of a sinusoidal alternating current excitation signal of the alternating current measuring bridge, demodulation error and demodulation noise of the phase-sensitive detection circuit are suppressed, and quantization noise in a signal pass band is suppressed.

2. The method of claim 1, wherein the step of measuring the ac bridge measurement circuit comprises the steps of: a digital decimation filter circuit is additionally arranged;

the signals output by the A/D conversion circuit in the increment-sum adjusting circuit are output by the digital decimation filter;

the digital decimation filter circuit adopts a low-pass filter for filtering the quantization error of the A/D conversion circuit and reducing the sampling rate to obtain high-resolution measurement data.

3. The method of claim 2, wherein the step of measuring the ac bridge measurement circuit comprises the steps of: the sampling frequency of the increment-sum modulation circuit is at least twenty times of the highest frequency of the measured signal;

the feedforward integration circuit is a second-order integration circuit with a transfer function ofWhen it is needed to satisfy The value of (A) is between 0.3 and 2.0;

the feedforward integration circuit is a third-order integration circuit with a transfer function ofThen, the following conditions are satisfied: (1)(2)the value of (A) is between 2.74 and 17.58; (3)is between 2.74 and 17.58, where a, b, and c are coefficients of the feedforward integration circuit;

wherein: s represents a complex variable, f_sIs the sampling frequency, a, b and c are the coefficients of a feedforward integration circuit, k_AIs the gain, k, of the amplifying circuit_DIs the detection gain, U, of a phase-sensitive detection circuit_RIs the amplitude, V, of the reference source input signal of the D/A conversion circuit_REFIs a reference source voltage of the a/D conversion circuit.

4. The method of claim 3, wherein the step of measuring the ac bridge measurement circuit comprises the steps of: the phase-sensitive detection circuit is a circuit designed based on an analog switch, a switch control signal of the phase-sensitive detection circuit is a square wave signal, and the frequency of the square wave signal is the same as that of an alternating current excitation signal source of the alternating current measuring bridge;

and the reference source of the D/A conversion circuit uses an alternating current signal with the same frequency and the same phase as the alternating current excitation signal source of the alternating current measuring bridge.

5. The method of claim 3, wherein the step of measuring the ac bridge measurement circuit comprises the steps of: the phase-sensitive detection circuit is a circuit designed based on an analog multiplier, and a reference signal of the phase-sensitive detection circuit is a sine wave signal with the same frequency as the alternating current excitation signal source;

and the reference source of the D/A conversion circuit uses an alternating current signal with the same frequency and the same phase as the alternating current excitation signal source of the alternating current measuring bridge.

6. The method for improving the measurement accuracy of the alternating current bridge measurement circuit according to claim 4 or 5, wherein the method comprises the following steps: when the alternating current measuring bridge is arranged outside a feedback loop, a signal output by the alternating current measuring bridge and a feedback signal generated by the D/A conversion circuit are input to the amplifying circuit after superposition operation;

when the alternating current measuring bridge is arranged in the feedback ring, the feedback signal generated by the D/A conversion circuit is input to the measuring signal input end of the alternating current bridge.

7. The method of claim 6, wherein the step of measuring the ac bridge measurement circuit comprises: the four arms constituting the ac measuring bridge may be transformer windings or inductors or capacitors or resistors.

Technical Field

The invention relates to an alternating current bridge measuring circuit and a method for improving the measuring precision thereof, in particular to a closed-loop alternating current bridge measuring circuit embedded with a phase-sensitive detection circuit and an increment-sum modulation circuit and a method for effectively inhibiting the demodulation error of the alternating current bridge measuring circuit, reducing the quantization noise and improving the measuring precision.

Background

In earthquake and geophysical observation instruments (such as seismometers, extensometers, borehole strain gauges, pendulum inclinometers, spring gravimeters and the like), differential capacitive sensors are widely used for sensing micro-displacement, i.e., converting the change of displacement into the change of capacitance, and then measuring the change of capacitance through an alternating current bridge measuring circuit, thereby obtaining the change of displacement. Some scopes also use a displacement sensor in the design of a differential transformer to sense micro-displacements, the measurement circuit of which is also typically an ac bridge measurement circuit.

FIG. 1 is a schematic block diagram of an AC bridge-based measurement circuit widely used in a conventional sensor, and the measurement circuit includes an AC measurement bridge 1, an AC signal driving source U_BAn amplifier circuit 2, a phase-sensitive detector circuit 3, and an A/D converter circuit 4.

AC signal driving source U_BThe AC measuring bridge 1 is supplied with operating power and has an output voltage ofWhen the AC measuring bridge is in equilibrium, i.e. Z₁Z₄＝Z₂Z₃While, its output voltage U_S0. When the change of the measured physical quantity causes the AC measuring bridge to deviate from the equilibrium state, the output voltage U thereof_SWill vary in amplitude with the phase corresponding to the direction of departure from the equilibrium state. Therefore, it is necessary to process the output signal Us of the AC measuring bridge amplified by the amplifying circuit using the phase-sensitive detection circuit 3 to obtain the amplitude sum U_SThe average value is proportional to the voltage signal whose polarity is related to the direction of bridge deviating from balance state, and said voltage signal is processed by A/D conversion circuit 4 and outputted.

The bridge circuit used in the sensor has the characteristic of high measurement sensitivity, but the measurement accuracy of the measurement circuit based on the alternating current bridge is not high due to the following factors: (1) the phase shift generated by the ac measuring bridge 1 and the amplifier circuit 2 causes a phase difference between the detected signal and the reference signal input to the phase-sensitive detector circuit 3, which lowers the detection gain of the phase-sensitive detector circuit 3. The problem of measurement errors caused by the phase-sensitive detection circuit is solved. For the phase-sensitive detection circuit based on the analog multiplier, the measurement error introduced by the phase-sensitive detection circuit 3 mainly comes from the zero voltage drift of the analog multiplier; for phase sensitive detectors based on analog switches, the measurement error introduced by the phase sensitive detection circuit 3 mainly comes from analog switch response speed and control signal leakage. And the nonlinear errors of the amplifying circuit 2 and the phase-sensitive detection circuit 3 are reduced. The a/D conversion circuit 4 generates a zero drift error, a gain error, a quantization error, and the like. The errors are superposed step by step, so that the difficulty of realizing high precision of the measuring circuit based on the alternating current bridge is increased, and the measuring precision of the whole alternating current bridge measuring circuit is reduced.

Disclosure of Invention

In view of the above, it is an object of the present invention to provide a method for effectively suppressing the demodulation error of an ac bridge measuring circuit, reducing the quantization noise, and improving the measurement accuracy of the ac bridge measuring circuit.

In order to achieve the purpose, the invention adopts the following technical scheme: a method for improving the measurement accuracy of an alternating current bridge measurement circuit is characterized in that: an increment-sum modulation circuit is additionally arranged in an original serially connected alternating current measuring bridge, an amplifying circuit and a phase sensitive detection circuit; the alternating current measuring bridge, the amplifying circuit, the phase-sensitive detection circuit and the increment-sum adjusting circuit are connected in series to form a negative feedback closed loop;

the increment-sum modulation circuit is composed of a feedforward integration circuit, an A/D conversion circuit and a D/A conversion circuit which are connected in series;

the signals output by the A/D conversion circuit in the increment-sum adjusting circuit are divided into two paths, and one path outputs digital signals; the other path of the signal is connected with a signal input end of a D/A conversion circuit in the increment-sum modulation circuit, a feedback signal is generated by the D/A conversion circuit and is fed back to a measurement signal input end of the alternating current measurement bridge or the input end of the amplifying circuit;

by adding an increment-sum modulation circuit in the measuring circuit and adopting higher gain of the amplifying circuit and amplitude of a sinusoidal alternating current excitation signal of the alternating current measuring bridge, demodulation error and demodulation noise of the phase-sensitive detection circuit are suppressed, and quantization noise in a signal pass band is suppressed.

The invention improves the method of measuring accuracy of the alternating current bridge measuring circuit, also include setting up the digital decimation filter circuit;

the signals output by the A/D conversion circuit in the delta-sigma modulation circuit are output by the digital decimation filter; the digital decimation filter circuit adopts a low-pass filter for filtering the quantization error of the A/D conversion circuit and reducing the sampling rate to obtain high-resolution measurement data.

In the preferred embodiment of the invention, the sampling frequency of the increment-sum modulation circuit in the alternating current bridge measuring circuit is at least twenty times of the highest frequency of the measured signal;

the feedforward integration circuit is a second-order integration circuit with a transfer function ofWhen it is needed to satisfy The value of (A) is between 0.3 and 2.0;

the feedforward integration circuit is a third-order integration circuit with a transfer function ofThen, the following conditions are satisfied: the value of (A) is between 2.74 and 17.58;the value of (A) is between 2.74 and 17.58;

wherein: s represents a complex variable, f_sIs the sampling frequency, a, b and c are the coefficients of a feedforward integration circuit, k_AIs the gain, k, of the amplifying circuit_DBeing phase-sensitive detection circuitsDetection gain, U_RIs the amplitude, V, of the reference source input signal of the D/A conversion circuit_REFIs a reference source voltage of the a/D conversion circuit.

In the preferred embodiment of the present invention, the phase-sensitive detection circuit is a circuit designed based on an analog switch, and the switch control signal is a square wave signal, and the frequency of the square wave signal is the same as the frequency of the ac excitation signal source of the ac measurement bridge;

and the reference source of the D/A conversion circuit uses an alternating current signal with the same frequency and the same phase as the alternating current excitation signal source of the alternating current measuring bridge.

In the preferred embodiment of the invention, the phase-sensitive detection circuit is a circuit designed based on an analog multiplier, and the reference signal of the phase-sensitive detection circuit is a sine wave signal with the same frequency as the alternating current excitation signal source of the alternating current measuring bridge;

and the reference source of the D/A conversion circuit uses an alternating current signal with the same frequency and the same phase as the alternating current excitation signal source of the alternating current measuring bridge.

In a preferred embodiment of the present invention, when the ac measurement bridge is disposed outside the feedback loop, the signal output by the ac measurement bridge and the feedback signal generated by the D/a conversion circuit are input to the amplifying circuit after being subjected to a superposition operation;

when the alternating current measuring bridge is arranged in the feedback ring, the feedback signal generated by the D/A conversion circuit is input to the measuring signal input end of the alternating current bridge.

Drawings

FIG. 1 is a schematic block diagram of a conventional AC bridge measurement circuit;

FIG. 2 is a schematic diagram of a closed loop AC bridge measurement circuit with embedded phase sensitive detection and delta-sigma modulation circuits according to the present invention;

FIG. 3 is a graph of magnitude response of the embodiment of the measurement circuit shown in FIG. 2 to an input AC bridge measurement, a phase sensitive detector circuit error signal, and an A/D converter circuit error signal;

FIG. 4 is a schematic diagram of a closed-loop AC bridge measurement circuit with embedded phase-sensitive detection and delta-sigma modulation circuits, which is applied to a differential capacitance displacement sensor according to embodiment 1 of the present invention;

FIG. 5 is a schematic diagram of another closed-loop AC bridge measurement circuit with embedded phase-sensitive detection and delta-sigma modulation circuits, as applied to a differential capacitance displacement sensor in accordance with embodiment 2 of the present invention;

FIG. 6 is a schematic diagram of a closed-loop AC bridge measuring circuit embedded with a phase-sensitive detection and increment-sum modulation circuit and applied to a variable-gap differential transformer displacement sensor in embodiment 3 of the invention;

FIG. 7 is a schematic diagram of a closed-loop AC bridge measurement circuit embedded with a phase-sensitive detection and delta-sigma modulation circuit applied to temperature measurement in embodiment 4 of the present invention;

FIG. 8 is a schematic diagram of a closed loop AC bridge measurement circuit of the present invention incorporating a phase sensitive detection circuit based on an analog multiplier design;

fig. 9 is a diagram of a phase sensitive detector circuit reference signal using an analog multiplier design.

Detailed Description

The technical features of the present invention will be described in further detail with reference to the accompanying drawings and examples.

As shown in fig. 2, the closed-loop ac bridge measuring circuit disclosed by the present invention adds an increment-sum modulation circuit on the basis of the existing ac bridge measuring circuit, and connects the phase-sensitive detection circuit and the increment-sum modulation circuit in series to form a negative feedback closed-loop ac bridge measuring circuit.

As shown in the figure, the increment-sum modulation circuit is composed of a feedforward integrating circuit 6, an A/D conversion circuit 4 and a D/A conversion circuit 7, the signal input end of the feedforward integrating circuit 6 is connected with the signal output end of the phase-sensitive detection circuit 3, the signal output end of the feedforward integrating circuit 6 is connected with the signal input end of the A/D conversion circuit 4, the signal output end of the A/D conversion circuit 4 is divided into two paths, one path is connected with the signal input end of the alternating current measurement bridge 1 through the D/A conversion circuit 7 to form negative feedback to form a closed loop, and the other path is connected with the signal input end of the digital extraction filter circuit 5.

The difference between the present invention and the traditional AC bridge measuring circuit is that: the invention forms a closed loop with negative feedback, and a phase-sensitive detection circuit and an increment-sum modulation circuit are embedded in the closed loop. The measurement precision and the resolution of the alternating current bridge digital measurement circuit are improved by a closed loop with negative feedback, a phase sensitive detection circuit and an increment-sum modulation circuit which are embedded in the closed loop, and the errors (such as gain errors and nonlinear errors generated by an amplifying circuit, the phase sensitive detection circuit and an A/D conversion circuit) introduced by each stage of circuits in the feedback loop, noise and zero drift generated by the phase sensitive detection circuit and zero drift generated by the A/D conversion circuit.

The advantages of the closed-loop ac bridge measurement circuit of the present invention with embedded phase sensitive detection circuit and delta-sigma modulation circuit are demonstrated by detailed theoretical derivation below.

As shown in FIG. 2, assume that the AC excitation drive power source U constituting the AC measurement bridge 1 of the present invention_B＝2U_msinω₀t, wherein U_mIs the amplitude, omega, of a sinusoidal AC excitation signal_oIs the angular frequency of the sinusoidal ac excitation signal. The digital value input of the D/A conversion circuit 7 is D, and the reference source input signal is U_Rsinω₀t, wherein U_RFor the amplitude, omega, of the input signal of the reference source_oWhen the angular frequency of the input signal of the reference source is the same as the angular frequency of the sinusoidal AC excitation signal, the output signal of the D/A conversion circuit 7 is-D.2^-N·U_Rsinω₀And t, wherein N represents the number of bits of the D/A conversion chip. The output signal of the D/a converter circuit 7 is connected as a feedback signal to the input of the ac measuring bridge. The equivalent impedance of the AC measuring bridge 1 isWhen the AC excitation signal of the AC measuring bridge 1 is U_B＝2U_msinω₀At t, the output voltage of the AC measuring bridge 1 is2U_msinω₀t-D·2^-NU_Rsinω₀t. Output voltage U of ac measuring bridge 1_SAmplified by the amplifier circuit 2 and inputted toA phase sensitive detection circuit 3.

The phase-sensitive detection circuit 3 may be formed by an analog switching circuit, and the switching control signal is a square wave signal having a period ofThe frequency (reciprocal of period) of this square wave signal is the same as the frequency of the ac excitation drive power source of the ac measuring bridge 1.

Phase sensitive detection circuit 3 for input signalCan be expressed as Wherein, U_ARepresenting the amplitude, phase angle, of the sine wave signal input to the phase-sensitive detection circuit, i.e. the amplitude, phase angle, of the sine wave signal output by the amplification circuit 2 from the output voltage Us of the AC measuring bridge 1The phase difference between the input signal of the phase sensitive detection circuit and the reference signal is referred to. At this time, the transmission coefficient (or detection gain) of the phase sensitive detection circuit isThat is, the output signal varies with the cosine of the phase difference, and when the phase difference is 0, the transmission coefficient of the circuit is

If the gain of the amplifying circuit is k_AThe transfer function of the feedforward integration circuit 6 is designed asWhere s represents a complex variable and a and b represent coefficients of a feedforward integration circuit. Phase-sensitive detection circuitThe gain of the path is k_DThe error signal generated by the phase sensitive detection circuit is denoted as e_MThe error signal of the A/D conversion circuit is denoted as e_QThe digital value 2 outputted by the circuit of FIG. 2^-ND pairs of AC bridge measurementHas a transfer function of

In the formula (1), s is a complex variable, which indicates that the closed-loop AC bridge measuring circuit embedded with phase-sensitive detection and delta-sigma modulation shown in FIG. 2 has the characteristic of low-pass filtering on the AC bridge measured quantity, and the passband gain is

Digital quantity 2 of circuit output of fig. 2^-ND phase-sensitive detection circuit error signal e_MHas a transfer function of

As can be seen from the equation (2), the closed-loop AC bridge measuring circuit with the embedded phase-sensitive detection and delta-sigma modulation circuit also exhibits the characteristic of low-pass filtering for the phase-sensitive detection error signal, but has different pass-band gains and measures the AC bridgePass band gain ofError signal e of phase-sensitive detection circuit_MPass band gain ofMuch smaller than the former, generally the magnification k_AAnd amplitude U of the sinusoidal AC excitation signal_mMuch greater than 1, k_DIs composed ofTherefore, the closed-loop alternating current bridge measuring circuit embedded with the phase-sensitive detection and the delta-sigma modulation has a good inhibiting effect on error signals of the phase-sensitive detection.

Digital quantity 2 of circuit output of fig. 2^-ND to A/D conversion circuit error signal e_QHas a transfer function of

As can be seen from equation (3), the closed-loop ac measurement circuit with embedded phase-sensitive detection and delta-sigma modulation shown in fig. 2 exhibits high-pass filtering characteristics for a/D conversion circuit errors (including quantization noise). As the geophysical signals concerned by people are concentrated in a low frequency band, the circuit has good inhibition effect on the errors of the A/D conversion circuit in a signal passband of the low frequency band; for the A/D conversion circuit error outside the signal pass band, the error can be filtered by a digital low-pass decimation filter. This conclusion is consistent with the delta-sigma (Δ -sigma) modulator theory, so that the circuit shown in fig. 2 can achieve higher resolution using a/D conversion circuits and D/a conversion circuits with lower bits.

From the reasoning, the invention can effectively inhibit the demodulation error and the demodulation noise of the phase-sensitive detection circuit by reasonably designing the parameters of the feedback loop and adopting higher amplification circuit gain and the amplitude of the sinusoidal alternating current excitation signal, and simultaneously effectively inhibit the quantization noise in a signal pass band, thereby allowing the use of ADC and DAC chips with lower digits and obtaining a digital measurement result with high resolution.

In the measuring circuit, the sampling frequency of the increment-sum modulation circuit is far higher than the measured signal and is at least twenty times higher than the highest frequency of the measured signal. The feed forward integrator circuit is second order and has a transfer function ofIn time, the selection of the parameters needs to satisfy the following conditions:wherein f is_sIs the sampling frequency;is between 0.3 and 2.0, where a and b are the coefficients of the feedforward integration circuit, k_AIs the gain, k, of the amplifying circuit_DIs the detection gain, U, of a phase-sensitive detection circuit_RIs the amplitude, V, of the reference source input signal of the D/A conversion circuit_REFIs a reference source voltage of the a/D conversion circuit. The feed forward integrator circuit is third order and has a transfer function ofIn time, the selection of the parameters needs to satisfy the following conditions:wherein f is_sIs the sampling frequency;the value of (A) is between 2.74 and 17.58;is between 2.74 and 17.58, where a, b, and c are coefficients of the feedforward integration circuit.

When the parameter is k_A＝10，U_m＝50V，V_REF＝10V，U_RWhen 20V, a 69.79 and b 31006.28 are used, the amplitude response curves of the measuring circuit to the input ac bridge measurement, the phase sensitive detector circuit error signal and the a/D converter circuit error signal can be obtained according to equations (1), (2) and (3), as shown by the solid line, the broken line and the dashed-dotted line in fig. 3. As can be seen, the measuring circuit presents the measuring quantity of the alternating current bridge and the error signal of the phase-sensitive detection circuitThe characteristic of low-pass filtering, but the gain of the pass band of the latter is obviously lower than that of the former, shows that the circuit has better inhibiting effect on error signals of a phase-sensitive detection circuit. The measuring circuit has the characteristic of high-pass filtering on an error signal of an A/D conversion circuit, and the geophysical signal concerned by people is concentrated in a low-frequency band, so that the measuring circuit has a good effect of inhibiting the error of the A/D conversion circuit in a signal pass band of the low-frequency band.

Examples 1 and 2

Fig. 4 and 5 are schematic circuit diagrams of embodiments of the differential capacitive micro-displacement sensor of the present invention for high-precision measurement. Differential capacitive sensors are a kind of conversion device that converts the measured physical quantity into a capacitance change, and are widely used for measuring displacement, strain, angle, vibration, velocity, pressure, etc.

As shown in FIGS. 4 and 5, the AC measurement bridge of the closed-loop AC bridge measurement circuit used in the differential capacitive sensor is composed of three parallel capacitor plates P₁、P₂、P₃And an excitation transformer T. The alternating current measuring bridge, the amplifying circuit, the phase-sensitive detection circuit, the feedforward integrating circuit, the A/D conversion circuit and the D/A conversion circuit form a closed loop feedback loop. When the alternating current measuring bridge is positioned in the feedback closed loop, a feedback signal output by the D/A conversion circuit is directly connected with a center tap of a secondary winding of an exciting transformer T of the alternating current measuring bridge; when the alternating current measuring bridge is positioned outside the feedback closed loop, the feedback signal output by the D/A conversion circuit and the measuring signal output by the alternating current measuring bridge are superposed and then input to the input end of the amplifying circuit.

Central capacitance polar plate P of differential capacitance sensor₂The output signal of the transformer is divided into two paths after passing through an amplifying circuit, a phase-sensitive detection circuit, a feedforward integrating circuit and an A/D converting circuit, one path of the output signal forms digital signal output after passing through a digital extraction filter, the other path of the output signal generates a feedback signal through the D/A converting circuit, and the feedback signal is connected to a central tap of a secondary winding of an excitation signal transformer T or is superposed with a measuring signal output by an alternating current measuring bridge and then is input to the input end of the amplifying circuit. Meanwhile, the other secondary winding of the excitation signal transformer T is in D/A conversionThe phase conversion circuit and the phase sensitive detection circuit provide a reference signal.

When the ac measurement bridge is placed in the feedback loop, the voltage across the secondary winding of the excitation signal transformer T is U, as shown in fig. 4_B＝2U_msinω₀t, the reference source input signal of the D/A of the digital-to-analog conversion circuit on the feedback loop is U_Rsinω₀t, digital input quantity is D, analog output signal is-D2^-N·U_Rsinω₀t, then adding to the two side plates P of the differential capacitor₁、P₃The voltage on is respectively (U)_msinω₀t-D·2^-N·U_Rsinω₀t) and (-U)_msinω₀t-D·2^-N·U_Rsinω₀t), central plate P of differential capacitor₂Output voltage of

In the formula C₁And C₂Representing the capacitance, ω, of two differential capacitors₀Is the angular frequency of the ac excitation signal. If the differential capacitance is a variable-pitch displacement measurement plate capacitor, the plate pitch is recorded as d, and the displacement is recorded as delta d

When the AC measuring bridge is placed outside the feedback loop, as shown in FIG. 5, the output voltage of the AC measuring bridge isCentral capacitance polar plate P of differential capacitance sensor₂The output signal is connected with the input end of the amplifying circuit, the digital signal after amplification, phase-sensitive detection, integration and A/D conversion is divided into two paths, one path forms output digital signal output through a digital extraction filter, the other path generates feedback signal through a D/A conversion circuit,the feedback signal is superposed with the signal output by the central capacitor plate P2 and then input to the amplifying circuit.

When the phase-sensitive detection circuit is realized by adopting an analog switch circuit, the switch control signal is a square wave signal, and the frequency of the square wave signal is the same as the frequency of an alternating current excitation signal source of the alternating current measuring bridge. Phase sensitive detection circuit for input signalCan be expressed asWherein U is_ARepresenting amplitude, phase angle of sine wave signal input to phase-sensitive detector circuitThe phase difference between the input signal of the phase-sensitive detection circuit and the reference signal is referred to. At this time, the transmission coefficient (or detection gain) of the phase sensitive detection circuit isThat is, the output signal varies with the cosine of the phase difference, and when the phase difference is 0, the transmission coefficient of the circuit is

The feedforward integration circuit 6, the a/D conversion circuit 4, and the D/a conversion circuit 7 constitute an incremental-sum modulation circuit that converts an analog signal output from the phase-sensitive detection circuit into a high-rate bit data stream with a high sampling frequency. The digital filter outside the feedback loop is a low-pass decimation filter used for filtering the quantization error of the A/D conversion circuit and reducing the sampling rate to obtain high-resolution measurement data.

When the gain of the amplifying circuit is k_AThe transfer function of the feedforward integration circuit 6 is designed asGain of phase sensitive detection circuit is k_DDemodulation by phase-sensitive detection circuitsError signal denoted as e_MThe quantization error of the A/D conversion circuit is denoted as e_QThe digital value (2) output by the circuits of fig. 4 and 5^-ND) For measuring AC bridgeThe transfer function of (a) is:

equation (5) shows that the closed-loop AC bridge measurement circuit applied in the differential capacitive sensor exhibits low-pass filtering characteristics for the AC bridge measurement with a pass-band gain of

Digital value (2) output by circuits in fig. 4 and 5^-ND) Demodulation error signal e of phase sensitive detection circuit_MHas a transfer function of

As can be seen from the equation (6), the closed-loop AC bridge measuring circuit applied to the differential capacitance sensor exhibits the low-pass filtering characteristic similarly to the phase-sensitive detection error signal, but has different pass-band gains, and the pass-band gain for the AC bridge measurement amount isPass band gain of error signal of phase sensitive detection circuit isMuch smaller than the former, only the formerIn general, the magnification k_AAnd the amplitude U of the sine AC excitation signal_mMuch greater than 1, k_DIs composed ofTherefore, the closed-loop alternating current bridge measuring circuit embedded with the phase-sensitive detection and the delta-sigma modulation has a good inhibiting effect on error signals of the phase-sensitive detection.

The circuits of fig. 4 and 5 output digital quantity (2)^-ND) To ADC conversion circuit error signal e_QHas a transfer function of

As shown in equation (7), the closed-loop AC bridge measurement circuit applied in the differential capacitive sensor exhibits a high-pass filtering characteristic for the A/D converter error signal. By implementing the sampling rate conversion and the low-pass filtering by the digital filter shown in fig. 4 and 5, the error noise signal of the a/D conversion circuit in the high frequency band can be filtered, that is, the circuit shown in fig. 4 and 5 can suppress the error (including quantization error) of the a/D conversion circuit in the signal pass band, thereby obtaining the measurement data with high resolution. This conclusion is consistent with the delta-sigma (Δ - Σ) modulator theory, so that the circuits shown in fig. 4 and 5 can achieve higher resolution using a/D and D/a chips with lower bits.

Example 3

FIG. 6 is a schematic circuit diagram of an embodiment of the present invention applied to a differential transformer micro-displacement sensor to achieve high-precision measurement. The differential transformer micro-displacement sensor is a sensor for converting measured non-electricity into mutual inductance change of sensor coil, and is made up according to the basic principle of transformer, and has important application in the measurement of stress, vibration, torque and flow quantity. The embodiment shown in fig. 6 is a variable gap differential transformer measurement circuit with a phase sensitive detector circuit and a delta-sigma modulator circuit embedded in the feedback loop.

W in FIG. 6_1a、W_2aTwo iron cores A, B of differential transformer respectively have primary coil turns W_1b、W_2bThe number of turns of the secondary coil of the two iron cores A, B of the differential transformer is the normal condition in practical casesW_1a＝W_2a，W_1b＝W_2b. The homonymous ends of the two primary windings of the differential transformer are connected in series in the forward direction, and the homonymous ends of the two secondary windings are connected in series in the reverse direction, so that differential output is formed. When the primary side coil is connected with an excitation voltage U_B＝2U_msinω₀After t, the secondary side coil will generate an induced voltage output. When the tested body has no displacement and the armature is at the initial balance position, the gap between the armature and the two iron cores is equal, the mutual induction potentials of the two secondary windings are equal, and the output voltage of the differential transformer is zero. The distance between the armature and the two cores when the armature is in the equilibrium position is denoted as d₀. When the tested body has displacement, the position of the armature connected with the tested body changes, mutual induction potentials of the two secondary windings are not equal any more, and the output of the differential transformer is not zero. Assuming that the armature has moved downward by a distance Δ d, the output of the differential transformer isThe magnitude of the output voltage is proportional to the magnitude of the displacement of the armature, and the phase is also related to the direction of movement of the armature.

The other secondary winding of the differential transformer core B provides a reference source for the D/A conversion circuit and the phase-sensitive detection circuit. Output signal U generated by secondary side coil of differential transformer_SThe digital signal output by the A/D conversion circuit is divided into two paths, one path of the digital signal is subjected to sampling rate conversion and low-pass filtering by a digital filter to obtain an output digital signal, and the other path of the digital signal is subjected to feedback voltage signal generation by the D/A conversion circuit and is connected with the input end of the amplification circuit to form a feedback closed circuit. The feedforward integrating circuit, the A/D converting circuit and the D/A converting circuit embedded in the closed-loop alternating current measuring circuit form an increment-sum modulating circuit.

The phase-sensitive detection circuit adopts an analog switch circuit, and the switch control signal is a square wave signal with the same frequency as the alternating current excitation signal source of the differential transformer. Phase sensitive detection circuit for input signalCan be expressed asWherein U is_ARepresenting amplitude, phase angle of sine wave signal input to phase-sensitive detector circuitThe phase difference between the input signal of the phase sensitive detection circuit and the reference signal is referred to. At this time, the transmission coefficient (or detection gain) of the phase sensitive detection circuit isThat is, the output signal varies with the cosine of the phase difference, and when the phase difference is 0, the transmission coefficient of the circuit is

The reference source input signal of the D/A conversion circuit is U_Rsinω₀T, D is the digital input quantity, the output signal of the D/A conversion circuit is-D.2^-N·U_Rsinω₀t, which is input to the amplifying circuit together with the output of the differential transformer.

When the gain of the amplifying circuit is k_AThe transfer function of the feedforward integral circuit is designed asGain of phase sensitive detection circuit is k_DThe demodulation error signal generated by the phase sensitive detection circuit is denoted as e_MThe quantization error of the A/D conversion circuit is denoted as e_QThe digital quantity (2) output can be obtained from FIG. 6^-ND) For measuring the differential transformerThe transfer function of (a) is:

as can be seen from the equation (8), the closed-loop AC bridge measuring circuit applied to the differential transformer displacement sensor has the characteristic of low-pass filtering the measured value of the differential transformer, and the pass-band gain is

The circuit of fig. 6 outputs a digital quantity (2)^-ND) Demodulation error e of phase sensitive detection circuit_MHas a transfer function of

As can be seen from the equation (9), the closed-loop AC bridge measuring circuit applied to the differential transformer displacement sensor exhibits the low-pass filter characteristic similarly to the phase-sensitive detection error signal, but has different pass-band gains, and the pass-band gain for the AC bridge measurement amount is set toPass band gain of error signal of phase sensitive detection circuit isSmaller than the former. In general, the magnification k_AAnd the amplitude U of the sine AC excitation signal_mMuch greater than 1, k_DIs composed ofTherefore, the closed-loop alternating current bridge measuring circuit embedded with the phase-sensitive detection and the delta-sigma modulation has a good inhibiting effect on error signals of the phase-sensitive detection.

The circuit of fig. 6 outputs a digital quantity (2)^-ND) For A/D conversion circuit error signal e_QHas a transfer function of

As can be seen from equation (10), the closed-loop ac bridge measuring circuit applied to the differential transformer displacement sensor exhibits a high-pass filtering characteristic for the error signal of the a/D conversion circuit. By implementing the sampling rate conversion and the low-pass filtering by the digital filter shown in fig. 6, the error noise signal of the a/D conversion circuit in the high frequency band can be filtered, that is, the circuit shown in fig. 6 can suppress the error (including quantization error) of the a/D conversion circuit in the signal passband, thereby obtaining the measurement data with high resolution. This conclusion is consistent with the delta-sigma (Δ - Σ) modulator theory, so that the circuit shown in fig. 6 can achieve higher resolution using a/D and D/a chips with a lower number of bits.

Example 4

FIG. 7 is a schematic diagram of an embodiment of the present invention applied to a thermometric sensor to achieve high-precision measurement, that is, an AC thermometric bridge measuring circuit with phase-sensitive detection and delta-sigma modulation embedded in a feedback loop.

In the AC bridge temperature measuring circuit shown in FIG. 7, the AC temperature measuring bridge is composed of a resistor R₁、R₂、R₃And R_TIn which R is₁、R₂And R₃Is a low-temperature drift precision resistor with fixed resistance value R_TThe temperature sensing element of the bridge can be a platinum resistor or a thermistor. When R is₁R_T＝R₂R₃The ac bridge is in equilibrium with zero output. When the temperature measured by the temperature sensing element changes, the resistance value of the temperature sensing element changes, which causes unbalanced voltage output at two ends of the bridge. When the excitation signal of the measuring bridge is U_B＝2U_msinω₀At t, the output of the AC temperature measuring bridge is

The output of the AC temperature measuring bridge is processed by an amplifying circuit, a phase-sensitive detection circuit, a feedforward integrating circuit and an A/D converting circuit, the signal after A/D conversion is divided into two paths, one path generates a feedback voltage signal through the D/A converting circuit, and the feedback voltage signal and the output of the temperature measuring bridge are added and then are sent to the amplifying circuitA circuit; and the other path of the digital signal is subjected to sampling rate conversion and low-pass filtering by a digital filter to obtain an output digital signal. The integrator, the A/D conversion circuit and the D/A conversion circuit in the feedback loop form a delta-sigma modulation circuit, wherein the reference source input signal of the D/A conversion circuit is U_Rsinω₀t, digital input quantity is D, output voltage is-D2^-N·U_Rsinω₀t。

The phase-sensitive detection circuit adopts an analog switch circuit, and the switch control signal is a square wave signal with the same frequency as the alternating current excitation signal source of the alternating current temperature measuring bridge. Phase sensitive detection circuit for input signalCan be expressed asWherein U is_ARepresenting the amplitude of the sine-wave signal input to the phase-sensitive detection circuit, i.e. the amplitude, phase angle of the sine-wave signal after the output Us of the AC thermometric bridge has passed through the amplification circuitThe phase difference between the input signal of the phase sensitive detection circuit and the reference signal is referred to. At this time, the transmission coefficient (or detection gain) of the phase sensitive detection circuit isThat is, the output signal varies with the cosine of the phase difference, and when the phase difference is 0, the transmission coefficient of the circuit is

When the gain of the amplifying circuit is k_AThe transfer function of the feedforward integral circuit is designed asGain of phase sensitive detection circuit is k_DThe demodulation error signal generated by the phase sensitive detection circuit is denoted as e_MQuantization error of ADCThe difference is represented as e_QFrom FIG. 7, the output digital value (2) can be obtained^-ND) For measurement of AC temperature measuring bridgeThe transfer function of (a) is:

as can be seen from the formula (11), the AC thermometric bridge measuring circuit with the phase-sensitive detection and delta-sigma modulation embedded in the feedback loop exhibits the characteristic of low-pass filtering on the AC thermometric bridge measured quantity, and the passband gain is

The circuit of fig. 7 outputs a digital quantity (2)^-ND) Demodulation error e of phase sensitive detection circuit_MThe transfer function of (a) is:

as can be seen from the equation (12), the AC thermometric bridge measuring circuit in which phase-sensitive detection and delta-sigma modulation are embedded in the feedback loop exhibits the same low-pass filtering characteristic for the phase-sensitive detection error signal, but has a different pass-band gain, and the pass-band gain for the AC bridge measurement is set toPass band gain of error signal of phase sensitive detection circuit isSmaller than the former. In general, the magnification k_AAnd the amplitude U of the sine AC excitation signal_mMuch greater than 1, k_DIs composed ofIt can be seen that the embedded phase sensitive detection and increment-totalAnd the modulated closed-loop alternating current bridge measuring circuit has good inhibition effect on error signals of phase-sensitive detection.

The circuit of fig. 7 outputs a digital quantity (2)^-ND) To ADC conversion circuit error signal e_QHas a transfer function of

As can be seen from equation (13), the ac thermometric bridge measurement circuit with embedded phase-sensitive detection and delta-sigma modulation in the feedback loop exhibits the characteristic of high-pass filtering on the error signal of the ADC conversion circuit. It is illustrated that the circuit shown in fig. 7 is capable of suppressing the error signal of the a/D conversion circuit in the signal pass band, including the quantization error. This conclusion is consistent with the delta-sigma (Δ - Σ) modulator theory, so that the circuit shown in fig. 7 can achieve higher resolution using a/D and D/a chips with a lower number of bits.

In the above example, the phase sensitive detector circuit is designed by using an analog switch circuit, and the phase sensitive detector circuit can also be designed by using an analog multiplier-based method as shown in fig. 9. When the phase sensitive detection circuit is designed by using an analog multiplier, the reference signal is a sine wave signal having the same frequency as the ac excitation signal source of the ac thermometric bridge, as shown in fig. 8. The input signal supplied to the phase-sensitive detection circuit is set toWherein U is_ARepresenting the amplitude of the sine-wave signal input to the phase-sensitive detection circuit, i.e. the amplitude, phase angle of the sine-wave signal after the output Us of the AC thermometric bridge has passed through the amplification circuitA phase difference between an input signal of a phase sensitive detection circuit and a reference signal is referred to. Let the reference signal of the phase-sensitive detection circuit be U_Jsinω₀t，U_JFor the amplitude of the reference signal, the output of the multiplier in FIG. 8 is

After passing through the low-pass filter, the high-frequency component of the second term in equation (14) is filtered out, leaving the first termWhen in useAt 0, the first term has a value ofThe transmission coefficient (detection gain) of the phase-sensitive detection filter based on the analog multiplier is expressed asFrom the foregoing analysis, it can be seen that the transmission coefficient of the phase-sensitive detector circuit based on the analog switch isThey differ by only a factor ofWhen the two are the same.

As can be seen from the analysis of the specific embodiments of the AC bridge measuring circuit with the phase-sensitive detection circuit and the delta-sigma modulation circuit embedded in the feedback loop in the differential capacitance sensor, the differential transformer sensor and the AC temperature measuring sensor, the closed-loop AC bridge measuring circuit of the invention has low-pass filter characteristics for the AC bridge measurement quantity and the phase-sensitive detection error signal, but has different passband gains, and the passband gain for the AC bridge measurement quantity isPass band gain of phase sensitive detection error signal isIs much smaller than the former, and is,of the former type only(U_mk_A＞＞1，) Therefore, the closed-loop alternating current bridge measuring circuit can effectively restrain the demodulation error signal generated by the phase-sensitive detection circuit. The closed-loop alternating current bridge measuring circuit has the characteristic of high-pass filtering on an error signal of an A/D conversion circuit, and has good inhibiting effect on the error of the A/D conversion circuit in a signal passband of a low frequency band because the frequency of physical quantity which needs to be observed is very low. For the error of the A/D conversion circuit outside the signal pass band, the error can be filtered by a digital low-pass decimation filter outside the feedback loop.

Aiming at the field of digitized measurement of the alternating current bridge, the negative feedback closed loop circuit formed by integrating the alternating current measurement bridge, the phase-sensitive detection circuit and the increment-sum modulation circuit can effectively inhibit demodulation errors and demodulation noise of the phase-sensitive detection circuit, reduce the noise level in a signal pass band, and effectively inhibit quantization errors introduced by an A/D (analog/digital) conversion circuit, allow the A/D and D/A chips with lower digits to realize high-resolution analog/digital conversion and data acquisition, and convert the tiny change of the measurement quantity of the alternating current bridge into high-quality digital signals to be output, so that the system has higher signal-to-noise ratio, larger dynamic range and stronger anti-jamming capability.

The above description is of the preferred embodiment of the present invention and the technical principles applied thereto, and it will be apparent to those skilled in the art that any changes and modifications based on the equivalent changes and simple substitutions of the technical solution of the present invention are within the protection scope of the present invention without departing from the spirit and scope of the present invention.