CN118191432A - A high-sensitivity microcapacitance detection device - Google Patents

Abstract

The present invention discloses a high-sensitivity micro-capacitance detection device. The present invention includes a capacitive displacement sensor probe and a micro-capacitance detection device. The capacitive displacement sensor probe converts the distance to be measured and the change between the probe and the object to be measured into a capacitance value and a capacitance change; the micro-capacitance detection device is used to detect the micro-capacitance value and the change of the capacitive displacement sensor probe, and convert the capacitance value and the change into a DC output. The micro-capacitance detection device includes a power supply and voltage reference generation module, a micro-capacitance detection core circuit module, an excitation signal generation module, a pre-conditioning circuit module, a phase-locked amplifier module, a multi-order low-pass filter module, and a gain and zero adjustment module. The present invention uses the virtual short characteristic of an operational amplifier as a method for realizing the equipotential requirements of the driving cable technology, and by setting the operational amplifier floating power ground to reduce the parasitic capacitance of the operational amplifier to the ground, it can realize high-precision and high-sensitivity micro-capacitance detection.

Description

A high-sensitivity microcapacitance detection device

Technical Field

The invention relates to the technical field of electronic measurement, and in particular to a high-sensitivity micro-capacitance detection device.

Background technique

The development of ultra-precision manufacturing and modern industrial production processes has given rise to an urgent need for high-precision and high-sensitivity micro-displacement detection. In addition to the friction-free and non-destructive characteristics of general non-contact sensing, capacitive displacement sensing technology also has the advantages of small size, low cost, high signal-to-noise ratio, high sensitivity, excellent dynamic response, and strong anti-electromagnetic interference ability. Therefore, it has received great attention. However, the background capacitance of small-sized capacitive sensors is mostly in the pF range, resulting in the capacitance change corresponding to the displacement change at the nanometer to micrometer scale being only in the aF to fF range. This tiny capacitance change is easily overwhelmed by the parasitic capacitance of the cable, the parasitic capacitance of the detection circuit, etc. and is difficult to detect. Therefore, how to improve the signal-to-noise ratio of micro-capacitance detection is a fundamental problem that must be solved. Constrained by the application scenarios of capacitive displacement sensors, the micro-capacitance detection circuit under this condition needs to overcome two key problems:

First, there is a certain distance between the capacitive sensor probe and the measurement circuit, so a coaxial shielded cable is required as the signal transmission cable. It is known that the parasitic capacitance of conventional coaxial cables is close to 100pF/m, which is far greater than the pF-level background capacitance of the sensor. Drive cable technology, also known as double-shielded equipotential transmission technology, is a common method to eliminate the influence of cable parasitic capacitance. However, the existing method of achieving equipotential through a voltage follower has extremely high requirements for the input capacitance, phase shift, and frequency band of the operational amplifier.

Second, the metal capacitive sensor probe is usually directly installed at the target detection position by screws, so one of the capacitor electrodes is often directly connected to the ground. At this time, the influence of parasitic capacitance to ground in the detection circuit needs to be overcome.

Summary of the invention

In order to solve the above problems existing in the existing tiny capacitance measurement technology, the present invention designs a tiny capacitance detection device based on the virtual short characteristic of an operational amplifier to achieve equipotential shielding, utilizes the virtual short characteristic of the operational amplifier when working in the linear region to achieve the equipotential requirement of the driving cable technology, and sets the operational amplifier power supply floating ground, which greatly reduces the influence of the operational amplifier-to-ground capacitance on the measurement, and can well improve the signal-to-noise ratio of tiny capacitance detection.

The invention comprises a capacitive displacement sensor probe and a micro-capacitance detection device.

Capacitive displacement sensor probe: converts the distance and change between the probe and the object to be measured into capacitance value and capacitance change;

Small capacitance detection device: used to detect the small capacitance value and change of the capacitance displacement sensor probe, and convert the capacitance value and change into a DC voltage output;

The micro capacitance detection device comprises:

The power supply and voltage reference module is used to provide the power supply and reference voltage signal required for the circuit operation;

The micro-capacitance detection core circuit module is used to convert the capacitance signal to be tested into an AC voltage signal related thereto;

An excitation signal generating module is used to provide the excitation signal required by the core circuit;

The pre-conditioning circuit module is used to amplify and filter the AC voltage signal output by the core circuit;

The phase-locked amplifier module is used to modulate and demodulate the output of the pre-conditioning circuit and convert the AC voltage output into a DC voltage output;

A multi-order low-pass filter module is used to perform low-pass filtering on the DC signal output by the phase-locked amplifier module;

The gain and zero adjustment module is used to perform DC gain adjustment and zero offset adjustment on the output signal.

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

By using the virtual short characteristic of the operational amplifier as the equipotential shielding effect required to achieve the driving cable technology, the performance requirements for the operational amplifier can be reduced compared to the traditional follower method of achieving equipotential.

By setting the ground of the op amp power supply to a floating ground and connecting it to the inner shielding layer of the driving cable, the effect of the op amp on the ground capacitance is well suppressed.

The micro capacitance detection device of the present invention can detect micro capacitance changes of the order of aF with high sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a capacitive displacement sensor according to an embodiment of the present invention;

FIG2 is a system overall framework diagram of a micro-capacitance detection device according to an embodiment of the present invention;

FIG3 is a core circuit diagram of a micro-capacitance detection device according to an embodiment of the present invention;

FIG4 is a comparison diagram of capacitance measurement values of a micro capacitance detection device according to an embodiment of the present invention and AH2700 bridge measurement values;

FIG. 5 is a fitting curve diagram of the output voltage value and the probe displacement of the micro-capacitance detection device according to an embodiment of the present invention.

Detailed ways

In order to make the description of the present invention clearer and easier to understand, the following embodiments and drawings are provided to further illustrate the present invention. It should be noted by those skilled in the art that the specific contents described below are protective and are not intended to limit the technical solutions of the present invention.

As shown in Figure 1, the structure diagram of the capacitive displacement sensor to be tested in this embodiment is a three-terminal capacitance structure design of the sensor sampling. The three electrodes from the inside to the outside are the center electrode, the guard electrode, and the shield electrode, which are respectively connected to the core wire, the inner shielding layer, and the outer shielding layer of the drive cable. There is a very thin insulating layer between each electrode. The sensor is provided with a three-coaxial cable connection port and a screw fixing hole.

This example provides an overall system framework of a micro capacitance detection device, as shown in FIG2 , including the following modules:

Module 1: The three-terminal capacitive displacement sensor is connected to the tiny capacitance detection core circuit in the detection device through a three-coaxial drive cable, wherein the core wire of the three-coaxial drive cable is connected to the negative input terminal of the operational amplifier that plays a measuring role in the core circuit, the inner shielding layer is connected to the positive input terminal of the operational amplifier, and the outer shielding layer is connected to the ground of the core circuit. The central electrode and the guard electrode in the three-terminal structure are virtually shorted by the operational amplifier in the core circuit to make them have the same potential, that is, the core wire of the drive cable has the same potential as the inner shielding layer, thereby meeting the equipotential shielding requirements of the drive cable technology. The capacitance to be measured is the tiny capacitance between the central electrode and the shielding electrode.

Module 2, the power supply and voltage reference generation module generates the power supply required for the operation of the circuit in the dotted box of FIG. 2 and the voltage reference required for the operation of some circuits.

In module three, the core circuit converts the change of the capacitance to be measured into the change of the protection electrode potential _VC-GND , and sets the power ground of the operational amplifier to a floating ground to suppress the influence of the operational amplifier's capacitance to ground on the measurement.

Module 4: The excitation signal generating circuit provides an excitation signal to the core circuit to determine the circuit operating frequency parameters.

Module five, the pre-conditioning circuit performs high-pass filtering on the _VC-GND potential signal (AC quantity) to improve the signal-to-noise ratio.

Module six, the phase-locked amplifier and low-pass filter convert the changes in the conditioned VC _-GND AC potential signal into more intuitive DC signal changes with a high signal-to-noise ratio.

Module seven, the multi-order filter further performs multi-order low-pass filtering on the above DC quantity to extract useful signals.

Module eight, the gain and zero adjustment module performs zero offset and gain adjustment on the above DC quantity to meet the output requirements.

As shown in FIG3 , this embodiment provides a core circuit of a micro-capacitance detection device for converting a capacitance signal into a voltage signal. The circuit consists of two parts: the first part is an AC excitation signal processing circuit, and the second part is a micro-capacitance change detection circuit. The two are connected in series through a filter capacitor C1 to filter out the influence of the DC component in the excitation signal.

In the first part, the AC excitation signal processing circuit includes an operational amplifier U1, the positive end of the operational amplifier U1 is connected to the adjustable end of the adjustable resistor R7, and the other two ends of the adjustable resistor R7 are respectively connected to one end of the resistor R1 and one end of the resistor R2; the other end of the resistor R1 is grounded, and the other end of the resistor R2 is connected to the protection electrode potential V _C-GND ; the negative end of the operational amplifier U1 is connected to the excitation signal VDAC through the resistor R3, and is also connected to the output end of the operational amplifier U1 through the resistor R4, and the power ground of the operational amplifier U1 is the floating ground C-GND. The first resistor R1, the second resistor R2 and the first adjustable resistor R7 are used to control the relationship between the protection electrode potential V _C-GND and the first potential point V1, and the third resistor R3 and the fourth resistor R4 are used to control the relationship between the first potential point V1, the second potential point V2 and the excitation signal VDAC.

In the second part, the micro capacitance change detection circuit includes an operational amplifier U2, the positive end of which is connected to the protection electrode potential _VC-GND ; the negative end of the operational amplifier U2 is respectively connected to one end of the filter capacitor C1, the central electrode and one end of the resistor R5, the other end of the resistor R5 is respectively connected to the output end of the operational amplifier U2 and one end of the resistor R6, the other end of the resistor R6 is grounded, and the power supply ground of the operational amplifier U2 is a floating ground C-GND. Wherein Cx is the capacitance to be measured between electrode 1 and electrode 3 (i.e., between the central electrode and the shielding electrode) in the three-terminal structure. As shown in Figure 3, the shielding electrode is directly connected to the earth; the central electrode is connected to the negative input terminal of the operational amplifier. In order to achieve the equipotential shielding effect, the remaining protection electrodes are connected to the positive input terminal of the operational amplifier U2, and the equipotential is achieved by using the virtual short characteristic of the operational amplifier. The fifth resistor R5, the sixth resistor R6, and the filter capacitor C1 determine the relationship between _VC-GND , the capacitance to be measured Cx and the third potential point V3. Since the output of the operational amplifier is grounded after passing through the resistor R6, the third potential point V3 is approximately equal to zero, so the measurement of the capacitance Cx to be measured can be converted into the measurement of V _C-GND .

The present invention sets the power supply ground of the two operational amplifiers in the core circuit to a floating ground C-GND that changes with the core line potential, and the positive and negative power supplies will change with the floating ground. The setting of the power supply floating ground can convert the operational amplifier to ground capacitance into capacitance to C-GND, and the capacitance will not be incorporated into the capacitance to be measured of the capacitive sensor. The core circuit is electrically connected to other circuit parts by grounding one end of the resistor R1 and the resistor R6. In addition, the circuit has another group of common power supplies, the ground of which is an analog ground, connected to the outer shielding layer of the triaxial cable, and this group of power supplies is applied to other active devices except the operational amplifier U1 and the operational amplifier U2.

The capacitance measurement value of the micro-capacitance detection device provided in this embodiment is compared with the capacitance measurement value of the AH2700 bridge as shown in FIG4. The capacitance measurement value of the micro-capacitance detection device in the figure is calculated based on the circuit relationship It is inferred from the output voltage value, where Rc _x is the capacitive reactance of the capacitor Cx to be measured. The comparison shows that the measurement results of the two devices are very close, and the differences are mainly attributed to factors such as residual parasitic capacitance effects and calculation errors. However, these differences do not affect the confirmation of the effectiveness of the detection circuit.

The relationship between the output voltage value and the probe displacement of the microcapacitance detection device provided in this embodiment is shown in FIG5. The displacement interval in FIG5 is a 200um linear change interval selected by accurately measuring the intervals with excellent linearity and different capacitance changes in FIG4, and the sensitivity in this interval is 4.4V/mm. When combined with a digital voltmeter with microvolt resolution, it can meet the resolution requirements for detecting nanometer displacement and aF-level capacitance change.

In summary, the present application proposes a highly sensitive micro-capacitance detection device, and in particular, relates to a method of using an operational amplifier virtual short circuit to achieve equipotential shielding, thereby solving problems such as cable parasitic capacitance. By setting the operational amplifier power ground of the core circuit to a floating ground, the influence of the operational amplifier-to-ground capacitance on the measurement is minimized, so that the measuring device of the present invention can achieve greater precision measurement.

The above is only a preferred embodiment of the present application and does not constitute any form of limitation to the present application. Any simple modification or equivalent changes made to the above embodiments based on the technical essence of the present application shall fall within the protection scope of the present application.

Claims (8)

1. A high sensitivity micro-capacitance detection device, comprising:

capacitive displacement sensor probe: converting the distance to be measured and the variation between the probe and the measured object into a capacitance value and a capacitance variation;

tiny capacitance detection device: the capacitance displacement sensor probe is used for detecting a tiny capacitance value and a change amount of the capacitance displacement sensor probe and converting the capacitance value and the change amount into direct-current voltage to be output; comprising the following steps:

the power supply and voltage reference module is used for providing power supply and reference voltage signals required by the operation of the circuit;

The micro capacitance detection core circuit module is used for converting a capacitance signal to be detected into an alternating voltage signal related to the capacitance signal;

the excitation signal generation module is used for providing excitation signals required by the core circuit;

The front conditioning circuit module is used for amplifying and filtering the alternating voltage signal output by the core circuit;

The phase-locked amplifying module is used for modulating and demodulating the output of the pre-conditioning circuit and converting the alternating voltage output quantity into direct voltage output quantity;

the multi-order low-pass filtering module is used for carrying out low-pass filtering on the direct current signal output by the phase-locked amplifying module;

And the gain and zero adjustment module is used for carrying out direct-current gain adjustment and zero offset adjustment on the output signal.

2. The high sensitivity micro-capacitance detection device according to claim 1, wherein the capacitive displacement sensor probe comprises:

the center electrode is positioned at the middle of the probe and is one pole of the capacitor to be tested and is connected with the core wire of the triaxial cable;

the protective electrode is positioned between the central electrode and the shielding electrode and is a main electrode for inhibiting edge effect in the three-terminal structure and is connected with the inner shielding layer of the triaxial cable;

The shielding electrode is positioned at the outermost layer of the probe and is the other electrode of the capacitor to be tested and is connected with the outer shielding layer of the triaxial cable;

And the coaxial cable connecting port is connected with the triaxial cable for signal transmission.

3. The high-sensitivity tiny capacitance detection device according to claim 2, wherein the tiny capacitance detection core circuit module comprises:

the alternating current excitation signal processing circuit processes the excitation signal and outputs the processed excitation signal to the micro capacitance change detection circuit;

the micro capacitance change detection circuit inputs a capacitance to be detected through a triaxial cable, converts a capacitance signal to be detected into a voltage signal through circuit processing and outputs the voltage signal;

The two are connected through a filter capacitor.

4. The high-sensitivity tiny capacitance detection device according to claim 3, wherein the alternating current excitation signal processing circuit comprises an operational amplifier U1, the positive end of the operational amplifier U1 is connected with the adjustable end of an adjustable resistor R7, and the other two ends of the adjustable resistor R7 are respectively connected with one end of a resistor R1 and one end of a resistor R2; the other end of the resistor R1 is grounded, and the other end of the resistor R2 is connected with the protection electrode potential V _C-GND; the negative end of the operational amplifier U1 is connected with an excitation signal VDAC through a resistor R3, and is also connected with the output end of the operational amplifier U1 through a resistor R4, and the power ground of the operational amplifier U1 is a floating ground C-GND.

5. The high-sensitivity tiny capacitance detection device according to claim 3, wherein the tiny capacitance change detection circuit comprises an operational amplifier U2, and the positive end of the operational amplifier U2 is connected with a protection electrode potential V _C-GND; the negative end of the operational amplifier U2 is respectively connected with one end of the filter capacitor C1, the center electrode and one end of the resistor R5, the other end of the resistor R5 is respectively connected with the output end of the operational amplifier U2 and one end of the resistor R6, the other end of the resistor R6 is grounded, and the power ground of the operational amplifier U2 is floating ground C-GND.

6. The high-sensitivity tiny capacitance detecting device according to claim 4 or 5, wherein positive and negative power supplies of the operational amplifier U1 and the operational amplifier U2 float together with a power supply ground, the ground of the positive and negative power supplies is a floating ground C-GND, and the voltage difference between the positive and negative power supplies to the ground is the same and constant.

7. The high-sensitivity tiny capacitance detection device according to claim 6, wherein the tiny capacitance detection device has two groups of power supplies, wherein the ground of one group of power supplies is floating ground C-GND, and is connected with an inner shielding layer of the triaxial cable, and the group of power supplies are only applied to an operational amplifier U1 and an operational amplifier U2; the ground of the other group of power supplies is analog ground and is connected with the outer shielding layer of the triaxial cable, and the group of power supplies are applied to other active circuits; the group power supply is electrically connected through a resistor R6 in the minute capacitance change detection circuit.

8. The device of claim 6, wherein the equipotential shielding effect required by the driving cable technology is realized by the virtual short characteristic of the operational amplifier in the linear region, wherein the capacitance signal to be measured and the processed excitation signal are input together to the negative end of the operational amplifier, and the positive end of the operational amplifier is connected with the inner shielding layer of the triaxial cable, and the potential of the inner shielding layer is V _C-GND, so that the variation of the capacitance to be measured is converted into the variation of the alternating voltage.