A capacitance sensor for measuring a non-grounded target plate

Abstract

The application discloses a kind of measurable non-ground target plate's capacitive sensor, belong to sensor technical field.The application includes: two current sources, for supplying alternating current, and the current direction of two current sources is opposite;Two current sources respectively excite two capacitive sensor circuits;Two probes, as the first electrode plate of capacitive sensor;The second electrode plate of capacitive sensor is the same conductor target plate;Second electrode plate and two first electrode plates form two to be measured capacitances;The target plate is not grounded, and stray capacitance is introduced between target plate and circuit ground;Current source forms voltage drop on the to-be-measured capacitance, and the size of the to-be-measured capacitance is reflected after the voltage drop is demodulated by filter circuit.The application designs double-probe structure, and cooperates with the same amplitude reverse current source circuit, realizes the capacitive measurement under the condition that target plate is not grounded, and is suitable for the occasion that target plate such as high-speed rotating shaft cannot be effectively connected.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sensor, in particular to a capacitive sensor capable of measuring non-grounded target plate. BACKGROUND

[0002] The capacitive sensor is a sensor based on the change relationship between the capacitance and other physical quantities. As can be seen from the capacitance calculation formula C = εS / d, the capacitance value C of the capacitor is related to the dielectric constant ε between the two capacitor plates, the plate area S and the plate spacing d. If two of them are fixed, the change of the capacitance value can reflect the change of the other quantity. For example, if the capacitor plates are fixed on two measured surfaces, the distance between the measured surfaces can be measured by measuring the capacitance value of the capacitor.

[0003] Generally speaking, the probe of the capacitive sensor is one plate, and the side surface is the other plate, and the two form a capacitor. Therefore, when using the capacitive sensor, the target plate must be metal, and the target plate must be connected to the circuit, generally connected to the circuit ground. However, in some measurement situations, such as measuring high-speed rotating parts such as shafts, the target plate cannot be grounded, resulting in the inability to form a capacitor and complete the measurement.

[0004] Chinese patent publication No. CN 1551988A discloses a sensor capacitive detection device and a sensor capacitive detection method. A capacitor C and an impedance converter H are inserted in series in the feedback circuit of a first operational amplifier OP1 iz The electrode P1 is connected through a signal line L at the connection point of the capacitor and the converter. The signal line L is connected to a reference potential through a high-resistance resistor R3. Although the signal line is in a suspended state when the capacitor is inserted in the feedback circuit, the circuit operation becomes unstable, but since the signal line L is fixed at a specified potential through the resistor R3, its operation can be stabilized. In addition, the impedance converter can also be formed by a voltage follower, and the resistor R3 is connected to its output. This application realizes accurate measurement of a small capacitor, and the principle of solving the problem is similar to the present application, but it cannot solve the problem that the target plate cannot be grounded in some measurement situations, resulting in the inability to form a capacitor and complete the measurement. SUMMARY

[0005] 1. Technical problem to be solved by the application

[0006] In view of the problems existing in the prior art, the present application provides a capacitive sensor capable of measuring non-grounded target plate. The present application designs a double-probe structure, which cooperates with a same-amplitude reverse current source circuit to realize capacitive measurement under the condition that the target plate is not grounded, and is suitable for occasions where the target plate such as high-speed rotating shaft cannot be effectively connected.

[0007] 2. Technical scheme

[0008] To achieve the above object, the technical scheme provided by the present application is as follows:

[0009] The capacitive sensor for measuring a non-grounded target plate comprises:

[0010] Two current sources for supplying alternating current, and the current directions of the two current sources are opposite; the two current sources respectively excite two capacitive sensor circuits;

[0011] Two probes, which are both the first electrode plates of the capacitive sensor; the second electrode plate of the capacitive sensor is the same conductor target plate; the second electrode plate and the two first electrode plates form two to-be-measured capacitances;

[0012] The target plate is not grounded, and a stray capacitance is introduced between the target plate and the circuit ground;

[0013] Two operational amplifiers;

[0014] Two demodulation filter circuits;

[0015] The current source excites the to-be-measured capacitance, and a voltage drop is formed on the to-be-measured capacitance, which reflects the size of the to-be-measured capacitance after the demodulation filter circuit.

[0016] Further, the current source is an alternating current source with constant amplitude, and the amplitudes of the two current sources are completely the same.

[0017] The capacitive sensor for measuring a non-grounded target plate comprises:

[0018] An excitation voltage U0;

[0019] A transformer T, which converts the excitation voltage into two paths with the same amplitude and opposite directions, and respectively excites two capacitive sensor circuits;

[0020] Two probes, which are both the first electrode plates of the capacitive sensor; the second electrode plate of the capacitive sensor is the same conductor target plate B; the second electrode plate and the two first electrode plates form two to-be-measured capacitances C x1 , C x2 ;

[0021] The target plate B is not grounded, and a stray capacitance C stray is introduced between the target plate B and the circuit ground;

[0022] Two operational amplifiers;

[0023] Two demodulation filter circuits;

[0024] The excitation voltage and the operational amplifier jointly form a voltage drop on the to-be-measured capacitance C x1 , C x2 , which reflects the size of the to-be-measured capacitance after the demodulation filter circuit.

[0025] Furthermore, the negative input terminals of the two operational amplifiers are connected to reference capacitors Cref1 and Cref2, respectively. The operational amplifiers pull the potential of their negative input terminals to ground via negative feedback, creating a voltage drop across the reference capacitors Cref1 and Cref2 identical to that of the AC voltage source. The AC voltage source, reference capacitors, and operational amplifiers together form a constant-amplitude AC current source. This current source is applied to the capacitor C under test. x1 C x2 A pressure drop is formed on top.

[0026] Furthermore, the reference capacitors Cref1 and Cref2 have the same capacitance value.

[0027] Furthermore, the two probes use the same capacitive sensor sensing element, and the probe is connected to the sensor main circuit via a coaxial line L.

[0028] 3. Beneficial effects

[0029] Compared with existing known technologies, the technical solution provided by this invention has the following significant advantages:

[0030] This invention discloses a capacitive sensor for measuring ungrounded target boards. By employing a configuration of opposite current sources of the same amplitude and dual capacitor probes, the current flowing from one current source can be completely absorbed by the other current source after passing through two capacitors. The current does not need to return from the target board to the measurement circuit, thus eliminating the need for any connection to the target board for measurement. Compared to conventional capacitive sensors, this invention is more convenient to use and suitable for applications such as high-speed rotating shafts where effective connection to the target board is impossible. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of an AC current source exciting a capacitive sensor.

[0032] Figure 2 for Figure 1 A specific circuit structure;

[0033] Figure 3 This is a schematic diagram of a capacitive sensor for a target board that does not require grounding in this invention.

[0034] Figure 4 for Figure 3 A schematic diagram of a specific implementation method;

[0035] Figure 5 This is a schematic diagram of a capacitive sensor structure that does not require grounding on the target board. Detailed Implementation

[0036] For further understanding of the present application, the present application is described in detail in conjunction with the accompanying drawings and examples.

[0037] Example 1

[0038] The capacitor sensor has various circuit implementation forms, among which the AC current source mode can directly output a voltage in linear relationship with the plate spacing. The circuit is shown in Figure 1 , a constant-amplitude AC current source outputs a current through the to-be-measured capacitor C x , which forms a voltage drop on the to-be-measured capacitor C x , and the voltage is input to the subsequent circuit for demodulation and output. The input impedance of the follower is much larger than the impedance of C x , so the shunt of the current source on the follower does not affect the measurement of the capacitor.

[0039] Among them, according to the formula C x = ε * S / d, the plate area S and the dielectric constant ε are constant values, and C x changes with the plate spacing d. The amplitude of the voltage drop formed by the constant-amplitude AC current source on the to-be-measured capacitor is , where all values are constant except d, and U is linearly related to d. The plate spacing can be obtained by detecting U. Figure 1 The upper plate of C x in the circuit is a capacitor sensor probe, and the lower plate is a target plate, which needs to be grounded.

[0040] Figure 2 is a specific circuit structure of Figure 1 , the operational amplifier pulls the potential at the negative input end to the instrument ground through negative feedback, thereby forming the same voltage drop on the reference capacitor Cref as the AC voltage source. The current through Cref is a fixed-amplitude AC current. At the same time, the input impedance of the operational amplifier is very large, and no shunt is formed at this point; the AC voltage source, the reference capacitor, and the operational amplifier together form a constant-amplitude AC current source, which forms a voltage drop on the to-be-measured capacitor C x . After demodulation and filtering, the voltage drop can reflect the size of the capacitor. However, in some measurement situations, such as measuring high-speed rotating parts such as shafts, the target plate cannot be grounded, resulting in the inability to form a capacitor and complete the measurement.

[0041] In order to solve the above problems, in combination with Figure 3 , the embodiment provides a capacitor sensor capable of measuring a non-grounded target plate, comprising:

[0042] Two current sources I1 and I2 are used to supply AC current, and the current directions of the two current sources are opposite; the two current sources respectively excite two capacitor sensor circuits;

[0043] Two probes, both as the first electrode plate of the capacitive sensor; the second electrode plate of the capacitive sensor is the same conductor target plate; the second electrode plate and the two first electrode plates form two to-be-measured capacitances;

[0044] The target plate B is not grounded, and a stray capacitance Cstray is introduced between the target plate and the circuit ground.

[0045] Two operational amplifiers;

[0046] Two demodulation filter circuits;

[0047] The current source and the operational amplifier jointly form a voltage drop on the to-be-measured capacitance, and the voltage drop is reflected on the size of the to-be-measured capacitance after the demodulation filter circuit.

[0048] When two opposite current sources are used, the upper plates of the two capacitances in the circuit are probes, and the lower plates are the same conductor target plate. The target plate is not grounded, and the stray capacitance between the target plate and the ground is Cstray. Since I1 and I2 are the same size and opposite directions, I3=0. Therefore, the voltage drop on the stray capacitance Cstray is zero, which can be equivalent to the target plate being grounded. For the two to-be-measured capacitances, U=2*π*I0*d / ε / S, (the phase difference is 180°) can be obtained respectively. The distance of the two probes to the target plate can be obtained, and the two outputs do not interfere with each other. Using the above structure can realize the displacement measurement of the non-grounded suspended target plate. In theory, when I1 and I2 are opposite in direction and completely the same in size, even if there is no stray capacitance, two channels can be completely measured independently, but in fact, this condition is difficult to achieve perfectly, therefore, a larger stray capacitance helps to improve the measurement accuracy.

[0049] In this embodiment, the same amplitude opposite current source and the double-capacitance probe configuration are adopted. The current flowing from one current source can be completely absorbed by another current source after passing through two capacitances. The current does not need to return to the measurement circuit from the target plate, so the target plate does not need to be connected for measurement. Compared with general capacitive sensors, the application is more convenient to use and is suitable for occasions where the target plate of a high-speed rotating shaft cannot be effectively connected.

[0050] Embodiment 2

[0051] Referring to Figure 4 In this embodiment, a transformer T is used to convert the excitation voltage into two paths of the same amplitude and opposite directions, which respectively excite two capacitive sensor circuits to form two same-amplitude opposite current sources. Two to-be-measured capacitances share one electrode, and a small stray capacitance exists between the electrode and the ground. In an ideal case, since the current flowing through C x1 and C x2The stray capacitance is zero, and there is no voltage drop. The target plate can be equivalent to ground, and the two independent capacitive measurement circuits are equivalent to two independent capacitive measurement circuits. When the two current sources cannot be strictly cancelled, a larger stray capacitance helps to reduce the voltage drop between the target plate and the ground, and improves the measurement accuracy.

[0052] Embodiment 3

[0053] In this embodiment, the sensor is designed as shown in the structure Figure 5 The probe contains two identical capacitive sensor sensitive elements, which are equivalent to two commonly used capacitive sensors placed side by side. The probe is connected to the sensor main circuit through a coaxial line L. The sensor circuit is excited by using current sources with opposite directions and the same amplitude. A large enough stray capacitance is introduced between the target plate and the circuit ground (the size of the stray capacitance required is different according to the accuracy). The data read from any probe can be used as the measured data.

[0054] The above description of the present application and its embodiments is illustrative and not limiting. The embodiments shown in the drawings are only one of the embodiments of the present application, and the actual structure is not limited thereto. Therefore, if a person skilled in the art is inspired by this, without departing from the spirit of the present application, similar structural forms and embodiments can be designed without creativity, which should belong to the protection scope of the present application.

Claims

1. A capacitive sensor for measuring a non-grounded target plate, characterized by Comprise: An excitation voltage U0; A transformer T, which converts the excitation voltage into two AC voltage sources of the same amplitude but in opposite directions, respectively exciting two capacitive sensor circuits; Two probes, both as first electrode plates of a capacitive sensor; the second electrode plate of the capacitive sensor is a same conductor target plate B; the second electrode plate and the two first electrode plates form two to-be-measured capacitances C x1 , C x2 , The two probes are placed side by side; The target plate B is not grounded, and a large enough stray capacitance C is introduced between the target plate B and the circuit ground stray to reduce the voltage drop between the target plate and the circuit ground; Two operational amplifiers; Two demodulation filter circuits; The excitation voltage and the operational amplifier jointly form a voltage drop on the capacitor C x1 , x2 The voltage drop is reflected by the size of the capacitor to be measured after being filtered by the demodulation circuit. The negative input terminals of the two operational amplifiers are connected to reference capacitors Cref1 and Cref2 respectively; the operational amplifiers pull the potential of the negative input terminals to the instrument ground through negative feedback, and form the same voltage drop on the reference capacitors Cref1 and Cref2 as the alternating voltage source, and the alternating voltage source, the reference capacitors and the operational amplifiers together form an alternating current source with constant amplitude, which forms a voltage drop on the capacitor C x1 under test x2 ; The capacitance values of the reference capacitors Cref1 and Cref2 are equal.

2. A capacitive sensor for measuring a non-grounded target plate according to claim 1, characterized in that: The two probes adopt the same capacitive sensor sensitive elements, and the two probes are connected to the sensor main circuit through coaxial lines L respectively.

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