CN111404503A - A Differential Capacitor AC Bridge Sensing and Controlling Circuit - Google Patents

Abstract

The invention provides a differential capacitance alternating current bridge sensing control circuit, which comprises: the alternating current excitation source is characterized in that 2 voltage-controlled operational amplifiers with reverse outputs are arranged at an alternating current signal output end to form a double-end differential output alternating current excitation source; the program-controlled DAC unit provides reference voltage for the voltage-controlled operational amplifier; by controlling the reference voltage, the amplitude of signals at two ends of the differential excitation source can be changed, so that the central point of the differential excitation is subjected to bidirectional offset; the differential capacitance sensor is composed of a three-pole capacitor, the distance between the pole plates at two sides is kept unchanged, and the middle pole plate can move towards two sides; two side electrode plates of the differential capacitance sensor are respectively connected with two ends of an alternating current excitation source of differential output to form an alternating current bridge; the alternating current bridge can reach a near-balance state by adjusting the central point of the differential excitation source; at the moment, the middle polar plate moves, so that the unbalanced voltage of the bridge caused by the movement is output after signal amplification, phase-sensitive detection and low-pass filtering, and the displacement of the polar plate in the capacitor is converted into a voltage signal.

Description

A Differential Capacitor AC Bridge Sensing and Controlling Circuit

technical field

The invention relates to the technical field of displacement measurement and differential AC bridge measurement, in particular to a differential capacitance AC bridge sensing measurement and control circuit.

Background technique

The differential capacitor AC bridge circuit is a high-precision measurement circuit, which is often used for displacement measurement. In the prior art, the balance adjustment and measurement and control of the bridge are generally realized by changing the ground point of the middle tap of the ratio transformer, or adjusting the distance between the plates of the capacitive sensor by mechanical means. The above adjustment methods all need to introduce special mechanical adjustment mechanisms, electronic switch components or power, which will increase the hardware cost of the equipment, and will introduce certain interference, affecting the accuracy of the measurement circuit.

SUMMARY OF THE INVENTION

Aiming at the defects of the prior art, a differential capacitor AC bridge sensing measurement and control circuit is proposed, which does not need to introduce a special mechanical adjustment mechanism and reduces the hardware cost.

A differential capacitance AC bridge sensing measurement and control circuit of the present invention includes: a differential AC excitation source, a program-controlled DAC unit, a voltage-controlled operational amplifier, a differential capacitance sensor and a modulation and demodulation circuit, wherein the differential AC excitation source For generating a sine wave signal with a required frequency, the two output ends of the differential AC excitation source are formed by the voltage-controlled operational amplifier, the program-controlled DAC unit provides a reference voltage for the voltage-controlled operational amplifier, and the The differential AC excitation source and the differential capacitance sensor form an AC bridge, and the output end of the differential capacitance sensor is connected to the modulation and demodulation circuit,

The program-controlled DAC unit changes the amplitude of the signal at both ends of the differential AC excitation source by controlling the reference voltage of the voltage-controlled operational amplifier, so that the center point of the differential AC excitation source is bidirectionally shifted, and the bridge reaches Balanced state; when the differential capacitance sensor generates a bridge unbalanced signal, the bridge unbalanced signal is output by the modulation and demodulation circuit, thereby converting the displacement of the polar plate in the capacitor into a voltage signal.

Preferably, two voltage-controlled operational amplifiers with opposite outputs are arranged at the AC signal output end of the differential AC excitation source to form a double-ended differential output AC excitation source.

Preferably, the program-controlled DAC unit includes a program-controlled component and a DAC element, and the program-controlled component controls the DAC element to provide a reference voltage for the voltage-controlled operational amplifier.

Preferably, the differential capacitance sensor is composed of three-pole capacitors, the two-side pole plates are fixed by ceramic spacers to keep the distance unchanged, and the middle pole plate can move to both sides, thereby generating an unbalanced signal of the bridge.

Preferably, the polar plates on both sides of the differential capacitive sensor are respectively connected to both ends of the AC excitation source of the differential output to form an AC bridge.

Preferably, the modulation and demodulation circuit includes a signal amplification circuit, a phase-sensitive detection circuit and a low-pass filter circuit.

Beneficial effects of the present invention: The present invention does not need to introduce a special mechanical adjustment mechanism, and only sets two voltage-controlled operational amplifiers with reverse output at the AC signal output end through program-controlled electronic components such as DAC to form a double-ended differential output AC source of motivation. The program-controlled unit provides a reference voltage for the voltage-controlled operational amplifier by controlling the DAC element, and changes the amplitude of the signals at both ends of the differential excitation source by controlling the reference voltage of the voltage-controlled operational amplifier, so that the center point of the differential excitation is bidirectionally shifted. The polar plates on both sides of the differential capacitive sensor are respectively connected with both ends of the AC excitation source of the differential output to form an AC bridge. The program-controlled unit can adjust the balance of the bridge by adjusting the output amplitude of the two voltage-controlled operational amplifiers with reverse outputs. At this time, when the middle plate of the capacitive sensor moves, the movement of the middle plate will cause the unbalance of the AC bridge, and the unbalanced voltage is output by the middle plate after signal amplification, phase-sensitive detection, and low-pass filtering, so that the capacitor can be neutralized. The displacement of the plate is converted into a voltage signal. Since it is not necessary to introduce a special mechanical adjustment mechanism, the hardware cost can be reduced, and no interference is introduced, and the precision of the measurement circuit can be improved.

Description of drawings

1 is a schematic structural diagram of a differential capacitor AC bridge sensing measurement and control circuit of the present invention;

FIG. 2 is a schematic structural diagram of an AC bridge of a differential capacitor AC bridge sensing measurement and control circuit of the present invention.

FIG. 3 is a schematic structural diagram of a differential capacitance sensor of a differential capacitance AC bridge sensing measurement and control circuit of the present invention.

4 is a schematic structural diagram of a program-controlled DAC unit of a differential capacitor AC bridge sensing measurement and control circuit of the present invention.

5 is a schematic structural diagram of a modulation and demodulation circuit of a differential capacitor AC bridge sensing measurement and control circuit of the present invention.

FIG. 6 is a schematic diagram of a phase-sensitive detection circuit of a differential capacitor AC bridge sensing measurement and control circuit of the present invention.

Detailed ways

The present invention will be further illustrated by the following examples, the purpose of which is only to better understand the research content of the present invention and not to limit the protection scope of the present invention.

FIG. 1 is a schematic structural diagram of a differential capacitor AC bridge sensing measurement and control circuit of the present invention. 2 is a schematic structural diagram of a program-controlled DAC unit of a differential capacitor AC bridge sensing measurement and control circuit of the present invention. FIG. 3 is a schematic structural diagram of an AC bridge of a differential capacitor AC bridge sensing measurement and control circuit of the present invention. FIG. 4 is a schematic structural diagram of a differential capacitance sensor of a differential capacitance AC bridge sensing measurement and control circuit of the present invention. 5 is a schematic structural diagram of a modulation and demodulation circuit of a differential capacitor AC bridge sensing measurement and control circuit of the present invention. FIG. 6 is a schematic diagram of a phase-sensitive detection circuit of a differential capacitor AC bridge sensing measurement and control circuit of the present invention. The structure of the present invention will be described in detail below with reference to FIGS. 1 to 6 .

As shown in FIG. 1 , the measurement and control circuit of the present invention includes a differential AC excitation source 1 , a program-controlled DAC (Digital Analog Converter) unit 2 , a voltage-controlled operational amplifier 3 , a differential capacitance sensor 4 and a modulation and demodulation circuit 5 .

Among them, the differential AC excitation source 1 is used to generate a sine wave signal of a required frequency, and the two output ends of the differential AC excitation source 1 are composed of two voltage-controlled operational amplifiers 3 . Specifically, two voltage-controlled operational amplifiers 3 with reverse output are arranged at the AC signal output end of the differential AC excitation source 1 to form an AC excitation source with double-ended differential output. The program-controlled DAC unit 2 provides a reference voltage for the voltage-controlled operational amplifier 3 . The differential AC excitation source 1 and the differential capacitive sensor 4 form an AC bridge. The differential capacitance sensor 4 is composed of three-pole plate capacitors. The pole plates 41 on both sides are fixed by ceramic spacers 43 with stable performance, so that the distance remains unchanged, and the middle pole plate 42 can move to both sides, thereby generating a bridge unbalanced signal. (see Figure 4). The polar plates 41 on the two sides of the differential capacitive sensor 4 are respectively connected with two ends of the AC excitation source 1 with differential output, thereby forming an AC bridge. The output end of the differential capacitance sensor 4 is connected to the modulation and demodulation circuit 5 .

Based on the above structure, the program-controlled DAC unit 2 changes the amplitude of the signal at both ends of the differential AC excitation source by controlling the reference voltage of the voltage-controlled operational amplifier 3, so that the center point of the differential AC excitation source 1 is bidirectionally shifted, and the bridge is balanced state. When the differential capacitance sensor 4 generates an unbalanced bridge signal due to the movement of the neutral plate, the unbalanced signal of the bridge is output by the modulation and demodulation circuit 5, thereby converting the displacement of the neutral plate of the capacitor into a voltage signal.

As shown in FIG. 2 , the differential AC excitation source 1 can be a high-precision signal generating unit (such as a SWR200 chip), and different capacitors can be connected in series as required to obtain a sine wave signal of the required frequency. The sine wave signal is output to the input terminals of the first voltage-controlled operational amplifier 31 and the second voltage-controlled operational amplifier 32 (eg VCA810 ) via the first operational amplifier 12 as a power amplifier. The signal is connected to the positive input terminal of the first voltage-controlled operational amplifier 31 and to the negative input terminal of the second voltage-controlled operational amplifier 32. After being amplified by the two voltage-controlled operational amplifiers 31 and 32, two reversed identical signals are output. The source AC signal is connected to the upper and lower plates of the differential capacitance sensor 4 respectively to form a capacitance bridge.

As shown in FIG. 3 , the differential capacitance sensor 4 is composed of three-pole capacitors. The two-side pole plates 41 are fixed by ceramic spacers 43 with stable performance, so that the distance d remains unchanged, and the middle pole plate 42 can move to both sides. As the displacement of the middle plate 42 forms a differential capacitance (the difference between C1 and C2), the voltage to ground output by the middle plate 42 is the reverse excitation and differential capacitance output by the two voltage-controlled operational amplifiers 31 and 32 The resulting bridge circuit is unbalanced in voltage. The voltage-controlled amplification of the two voltage-controlled operational amplifiers 31 and 32 (VCA810) is controlled by the reference voltages VG0 and VG1. By changing the two reference voltages of VG0 and VG1, the output voltage can be in the range of 0 to 5V. Variety.

As shown in FIG. 4 , the program-controlled DAC unit 2 is composed of a program-controlled component 21 and a DAC element 22 . The reference voltage VG0, VG1 signals are generated by the DAC element 22 (eg chip LTC2666). The LTC2666 can be a numerically controlled DAC, which is controlled by a process control component 21 (eg, a single-chip microcomputer 89C2051). The one-chip computer sends the instruction to LTC2666 through SPI bus line, makes VG0, VG1 produce the output change of 0- 5V/DC. The single-chip microcomputer is connected to the user terminal through the TTL serial port.

As above, by adjusting the output voltage amplitudes of the two voltage-controlled operational amplifiers 31 and 32, the bridge circuit can be balanced, that is, the output voltage of the neutral plate 42 is zero, so as to achieve the purpose of balance adjustment of the capacitor bridge. In one embodiment, in order for the balance adjustment not to affect the sensitivity of the measurement circuit, the total voltage applied across the sensor 4 may be constant at 5V. The reason why it is set to 5V is that the supply voltage of the two voltage-controlled operational amplifiers 31 and 32 (VCA810) is ±5V, so the maximum voltage output on one side cannot exceed 5V. In addition, since the rated input operating voltage of the DAC element 22 (LTC2666) is also 5V, the voltage-controlled operational amplifiers 31 and 32 (VCA810) also use a 5V supply voltage in order to match each other.

After the bridge circuit is balanced, along with the displacement of the neutral plate 42 , the unbalanced signal of the bridge circuit is output through the modulation and demodulation circuit 5 to convert the displacement of the capacitor neutral plate into a voltage signal. As shown in FIG. 5 , the modulation and demodulation circuit 5 includes a signal amplification circuit 51 , a phase-sensitive detection circuit 52 and a low-pass filter circuit 53 . The amplifier circuit 51 may be a two-stage amplifier circuit.

FIG. 6 is a schematic diagram of the phase-sensitive detection circuit 52 . The two-stage amplified unbalanced signal output by the capacitor middle plate 42 is divided into a reverse signal by the second operational amplifier 54, and is connected with Q1 and Q2 to form a phase-sensitive detection circuit. The phase shift circuit module 55 and the comparator circuit 56 provide a reference signal for the phase sensitive detection circuit. The phase shift circuit module 55 extracts the signal Yi from the excitation source output by the first operational amplifier 12, and performs phase shift processing to make the reference signal and the detection signal in phase. The comparator circuit 56 converts the phase-shifted sinusoidal signal of the phase-shifting circuit module 55 into two inverse square waves, controls Q1 and Q2 to form a gated multiplication circuit, and converts the sinusoidal signal into a half-wave sinusoidal signal. Finally, the second-order low-pass filter circuit 53 filters out the AC component of the signal, retains the DC amplitude information of the signal, and outputs it by a follower (not shown), thereby converting the displacement of the electrode plate in the capacitor into a voltage signal.

Obviously, those skilled in the art should realize that the above embodiments are only used to illustrate the present invention, not to limit the present invention. Variations and modifications of the examples will fall within the scope of the claims of the present invention.

Claims (6)

1. A differential capacitance AC bridge sensing measurement and control circuit is characterized in that, comprising: a differential AC excitation source, a program-controlled DAC unit, a voltage-controlled operational amplifier, a differential capacitance sensor and a modulation and demodulation circuit, wherein, The differential AC excitation source is used to generate a sine wave signal with a required frequency, the two output ends of the differential AC excitation source are composed of the voltage-controlled operational amplifier, and the program-controlled DAC unit is the voltage-controlled operational amplifier providing a reference voltage, the differential AC excitation source and the differential capacitance sensor form an AC bridge, and the output end of the differential capacitance sensor is connected to the modulation and demodulation circuit, The program-controlled DAC unit changes the amplitude of the signal at both ends of the differential AC excitation source by controlling the reference voltage of the voltage-controlled operational amplifier, so that the center point of the differential AC excitation source is bidirectionally shifted, and the bridge reaches Balanced state, When the differential capacitance sensor generates a bridge unbalanced signal, the bridge unbalanced signal is outputted by the modulation and demodulation circuit, so as to convert the displacement of the polar plate in the capacitor into a voltage signal. 2 . The differential capacitor AC bridge sensing measurement and control circuit according to claim 1 , wherein two voltage-controlled operational amplifiers with reverse outputs are provided at the AC signal output end of the differential AC excitation source, 2 . An AC excitation source that forms a double-ended differential output. 3. The differential capacitor AC bridge sensing measurement and control circuit according to claim 2, wherein the program-controlled DAC unit comprises a program-controlled component and a DAC element, and the program-controlled component controls the DAC element to be the voltage-controlled type The op amp provides the reference voltage. 4. The differential capacitance AC bridge sensing measurement and control circuit according to claim 1, wherein the differential capacitance sensor is composed of three-pole plate capacitors, and the pole plates on both sides are fixed by ceramic spacers, so that the distance is kept constant. Change, the middle plate can move to both sides, resulting in a bridge unbalanced signal. 5 . The differential capacitance AC bridge sensing measurement and control circuit according to claim 4 , wherein the two side plates of the differential capacitance sensor are respectively connected to both ends of the AC excitation source of the differential output to form an alternating current. 6 . bridge. 6 . The differential capacitor AC bridge sensing measurement and control circuit according to claim 1 , wherein the modulation and demodulation circuit comprises a signal amplification circuit, a phase-sensitive detection circuit and a low-pass filter circuit. 7 .

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