CN113566887B - Suspension motor rotor suspension position, deflection angle and rotating speed integrated detection system and application - Google Patents

Abstract

The invention relates to an integrated detection system and application for the suspension position, deflection angle and rotational speed of a suspension motor rotor, and belongs to the technical field of detection instruments. The system includes a radial test piece, an eddy current sensor, a preconditioner and a processor, wherein the eddy current sensor is connected with a preconditioner, the preconditioner is connected with a processor, and the preconditioner is used for powering the eddy current sensor and detecting and analyzing the circuit , the processor is used to process the measurement results of the eddy current sensor. The technical problems of complex structure of the rotor detection device, poor detection accuracy of measuring position and rotational speed, and low resolution in the prior art are overcome.

Description

A suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system systems and applications

technical field

The invention relates to an integrated detection system and application for the suspension position, deflection angle and rotational speed of a suspension motor rotor, and belongs to the technical field of detection instruments.

Background technique

The suspension motor has the advantage of no mechanical friction, which is more conducive to high-speed and high-efficiency operation. The stable and reliable operation of the active suspension motor requires real-time detection and closed-loop control of the rotor radial suspension position and rotation speed (even if the control method without speed sensing is used, the algorithm debugging stage still requires an external speed sensor to assist verification). At present, the detection of rotor radial vibration (position) usually adopts capacitive, inductive and eddy current sensors, and usually only detects the vibration in one direction of x or y (ignoring the vibration and displacement in its orthogonal direction), while the speed detection Encoders, resolvers, eddy currents, etc. are often used, and discrete sensor components are used to detect the suspended position and speed respectively. In order to avoid mutual interference, the installation size between the discrete sensors needs to be increased, which will lead to the axial direction of the motor. The increase in size reduces its critical speed and stability at high speeds.

Eddy current sensors have the advantages of strong measurement signal, wide dynamic response range, good reliability, and wide temperature adaptation range, and are often used for non-contact position measurement of plane and cylindrical surfaces. The eddy current sensor is arranged along the radial direction of the rotor to directly detect the radial position vibration of the rotor. After the modified stepped gear disc sleeve is rotated on the rotating shaft together with the rotating shaft, the eddy current sensor can be used to detect the rotor speed. However, there is no method to simultaneously realize the radial position and rotational speed detection, and for the suspension motor, due to the uncertainty of the suspension (eccentric) position of the rotor shaft, it is impossible to ensure that the sensor probe is always facing the outer normal direction of the cylindrical surface of the shaft to be measured. Therefore, the displacement change in the direction orthogonal to the detection direction of the probe will inevitably lead to a large uncertain detection error in the detection direction, which will affect the detection accuracy of the suspension position and rotation speed at the same time, especially after the integration of the two, the following problems are more obvious:

1. The uncertain change of the suspension position leads to the existence of eccentricity in the direction and position orthogonal to the direction to be measured, which affects the detection accuracy of the suspension position;

2. Uncertain changes in the floating position lead to the existence of eccentricity in the direction orthogonal to the direction to be measured, which affects the amplitude of the detection signal, resulting in high and low voltage pulse counting errors in the post-processing circuit of the rotational speed detection, which affects the accuracy of rotational speed detection. Spend;

3. At high rotational speed, the size and depth of the detection surface affect the detection sensitivity, resulting in poor sensitivity and low resolution.

For the above problems, there is no better solution at present.

SUMMARY OF THE INVENTION

In view of the deficiencies of the prior art, the present invention provides an integrated detection system for the suspension position, deflection angle and rotational speed of the rotor of a suspension motor, which overcomes the complex structure of the rotor detection device in the prior art, poor detection accuracy of the measurement position and rotational speed, and low resolution. technical problem.

The present invention also provides the application of the above-mentioned integrated detection system for the suspension position, deflection angle and rotational speed of the suspension motor rotor. The technical scheme of the present invention is as follows:

An integrated detection system for the suspension position, deflection angle and rotational speed of a suspension motor rotor comprises a radial test piece, an eddy current sensor, a pre-conditioner and a processor, wherein the eddy-current sensor is connected with a pre-conditioner, and the pre-conditioner is connected with a pre-conditioner The processor and the pre-processor are used to supply power to the eddy current sensor and detect and analyze the circuit. The processor is used to process the measurement results of the eddy current sensor and eliminate the influence of the eccentricity of the rotor radial suspension position on the accuracy of the detection result.

Preferably, the radial test piece comprises an annular cylinder, a circular arc segment is arranged on the outer side of the annular cylinder symmetrically, and the circular arc segment is concentric with the annular cylinder.

Further preferably, the circular arc segment is provided with at least 2 segments, and the more segments, the higher the frequency of the square wave signal obtained by one rotation of the rotor, and the higher the test sensitivity. In order to ensure dynamic balance and improve resolution, two stages are preferred.

Preferably, the radial test piece is a conductor material, hard aviation aluminum.

Preferably, the eddy current sensor is a high-frequency reflection type eddy current sensor, and the range and sensitivity can be selected according to different applications.

Preferably, the eddy current sensor is fixed to the outer casing of the suspension motor through the annular casing, and the eddy current sensor is threadedly connected to the annular casing and the outer casing of the suspension motor.

Preferably, the pre-conditioner includes an oscillator, a filter, a converter and a voltage stabilizer, and the eddy-current sensors are all connected with the pre-conditioner, and are calibrated and linearly compensated for the material and shape of the DUT before use. Detect the equivalent inductance change of the coil in the probe caused by the position change of the DUT, and convert it into the corresponding voltage. The voltage is proportional to the change in the distance between the measured surface and the probe, respectively.

Preferably, the processor includes a differential circuit, a summation circuit and a pulse counter for processing the voltage signal output by the eddy current sensor and the corresponding pre-conditioner;

The differential circuit eliminates the common mode interference signal. The source of the probe detection voltage deviation is mainly because the probe is not aligned with the axis of the rotor shaft, that is, the position eccentricity of the probe in the orthogonal direction; and for the two probes in opposite directions, the size of the eccentricity in the orthogonal direction is the same, so the impact on the output voltage of the probe when testing the displacement in this direction is the same (ie common mode interference). The differential circuit is used to collect the voltage signals representing the displacement in the direction collected by the two probes in the opposite direction. Subtraction can eliminate the influence of this part of the error, that is, common mode interference;

The summation circuit eliminates the influence of the rotor eccentricity on the high and low voltages of the speed detection, and ensures that the voltage signal representing the displacement in the detection direction remains unchanged when the rotor in the orthogonal direction of the detection direction is offset.

The summation circuit outputs the voltage to the pulse counter, and the pulse counter is used for speed measurement. According to the number (frequency) of high-voltage pulses per unit time, the current motor speed is obtained.

The application of the above-mentioned integrated detection system for the suspension position, deflection angle and rotational speed of the suspension motor rotor is as follows:

(1) Four eddy current sensors are arranged on the housing using a differential structure. The two opposite eddy current sensors are grouped together. The four eddy current sensors are spaced 90° apart in the plane. The probes of the eddy current sensors are installed in the direction of the rotor. The center line is used to detect the displacement of the outer surface of the radial test piece, and the displacement of the radial test piece is the rotor displacement;

(2) The radial test piece is installed and fixed on the rotor shaft by interference fit, so that the radial test piece rotates coaxially with the rotor;

(3) Start the suspension motor, and the probe of the eddy current sensor collects the information on the outer surface of the rotor in the opposite direction. During the rotation of the rotor, the shape of the outer surface of the rotor facing the probe changes continuously, and the probe collects a series of high and low voltage signals. Due to the constant change of the rotor displacement in the direction facing the probe, the displacement measured by the two probes in the opposite direction is also constantly changing, and there is no regularity (square waves with changing maximum and minimum values, even not square waves), resulting in inability to Directly perform rotational speed detection (as shown in Figure 1, when there is a certain and fixed displacement in the measured direction and the orthogonal direction, the change of the signals detected by the two eddy current sensors in the opposite direction). By adding the voltage signals collected by the eddy current sensors in opposite directions, a voltage signal with a constant amplitude can be obtained, and by frequency counting the voltage signal, the rotor speed can be obtained.

Preferably, in step (1), the zero position adjustment is performed when the eddy current sensor is installed. After the suspension motor is processed and assembled, it is subjected to the gravity of the rotor during static state, and the rotor is not at the radial center position, and the radial test piece is fixed by the zero adjustment tool. , make the radial test piece coaxial with the rotor, the eddy current sensor probe is calibrated for the material and shape of the radial test piece in advance, and the proportional coefficient between the radial test piece displacement and the voltage is obtained, and the measurement is based on the voltage obtained by the processor. The displacement in the direction of the eddy current sensor can be obtained by the signal value, and the current floating position of the rotor can be determined according to the displacement;

Then adjust the initial position of the eddy current sensor probe through the thread, and complete the zero position adjustment when the output voltage of the propulsion device is zero. Complete the zero adjustment of the remaining eddy current sensors in the same way.

Further preferably, the zero-adjustment tooling includes a hollow cylinder with one end open, a cylinder is arranged at the center of the bottom of the hollow cylinder, the inner diameter of the hollow cylinder is the same as the outer diameter of the annular shell, and is used for fixing the annular shell. The same as the rotor shaft diameter, the radial test piece is fixed on the cylinder during use, and the center position of the radial test piece is guaranteed to be the same as the center position during operation by the zero-adjusting tooling, so as to adjust the zero position of the eddy current sensor.

Preferably, in step (1), four eddy current sensors are arranged at both ends of the rotor to determine the axial installation distance L of the eddy current sensors at both ends, and the displacement difference (y1-y2) measured by the eddy current sensors in the corresponding directions at both ends The ratio of ) to the axial installation distance (L) is the tangent value tanα of the deflection angle of the rotor in this direction, and the deflection angle α of the rotor can be obtained by calculating the arc tangent.

The beneficial effects of the present invention are:

1. The design of the radial test piece of the present invention does not affect the dynamic balance of the original rotor, and at the same time ensures that the detection result of a single probe is a square wave with a duty ratio of 0.5, which provides convenience and foundation for the subsequent processor to realize the detection of the radial suspension position and rotational speed, The technical problems of complex structure of the rotor detection device, poor detection accuracy of measuring position and rotational speed, and low resolution in the prior art are overcome.

2. The four eddy current sensor probes of the present invention are arranged symmetrically at 90° in the same plane, which avoids the mutual influence and interference between the probes.

3. The present invention uses differential circuits to process eddy current sensor signals in opposite directions, eliminates common mode interference caused by eccentricity in orthogonal directions, and improves the detection accuracy and sensitivity of radial suspension positions.

4. The present invention uses a summation circuit to process eddy current sensor signals in opposite directions, eliminates the problem that the high and low voltages are different due to the eccentricity of the orthogonal direction, and the square wave signal is not obvious, and improves the speed detection accuracy and sensitivity.

5. The zero adjustment tooling of the present invention solves the problem that the initial zero position of the traditional suspension motor is difficult to adjust

Description of drawings

Fig. 1 is the output voltage diagram of each part when the initial position and the measured direction of the present invention are displaced.

FIG. 2 is a schematic diagram of the detection system of the present invention.

FIG. 3 is a positional arrangement diagram of the eddy current sensor of the present invention.

FIG. 4 is a structural diagram of the zero-adjustment tooling of the present invention.

Fig. 5 is the principle and signal flow diagram of the pre-processor of the present invention.

FIG. 6 is a schematic diagram of the internal oscillator + filter circuit of the preamplifier of the present invention.

FIG. 7 is a schematic diagram of the internal rectification converter circuit of the preamplifier of the present invention.

FIG. 8 is a schematic diagram of the differential circuit of the present invention.

Fig. 9 is the summation circuit principle diagram of the present invention.

FIG. 10 is a schematic diagram of the deflection angle calculation principle of the present invention.

Among them: 1. Rotor; 2. Radial test piece; 3. Eddy current sensor; 4. Preprocessor; 5. Processor; 6. Probe; 7. Ring-shaped shell; 8. Zero adjustment tool; 9 , arc segment.

Detailed ways

The present invention will be further described below with reference to the embodiments and the accompanying drawings, but is not limited thereto.

Example 1:

As shown in Figures 2-3, the present embodiment provides an integrated detection system for the suspension position, deflection angle and rotational speed of the rotor of the suspension motor, including a radial test piece 2, an eddy current sensor 3, a preconditioner 4 and a processor 5, Among them, the eddy current sensor 3 is connected with a pre-processor 4, the pre-processor 4 is connected with a processor 5, the pre-processor 4 is used to supply power to the eddy current sensor 3 and the detection and analysis circuit, and the processor 5 is used to process the eddy current sensor. The measurement results can eliminate the influence of the eccentricity of the rotor radial suspension position on the accuracy of the detection results.

The radial test piece 2 includes an annular cylinder, and a circular arc segment 9 is arranged symmetrically on the outer side of the annular cylinder, and the circular arc segment is concentric with the annular cylinder.

There are at least 2 segments in the arc segment. The more segments, the higher the frequency of the square wave signal obtained by the rotor rotating once, and the higher the test sensitivity. In order to ensure dynamic balance and improve resolution, two stages are preferred.

Radial test piece 2 is conductor material, hard aviation aluminum.

The eddy current sensor 3 uses a high-frequency reflection type eddy current sensor, and the range and sensitivity can be selected according to different applications.

The eddy current sensor 3 is fixed to the outer casing of the suspension motor through the annular casing 7, and the eddy current sensor is screwed through and connected to the annular casing and the outer casing of the suspension motor.

The pre-processor 4 includes an oscillator, a filter, a converter and a voltage stabilizer. The pre-processor is an existing device, the structure is shown in Figure 5, and the circuit connection diagram is shown in Figure 6-7. The eddy current sensors are connected with The pre-conditioner is calibrated and linearly compensated for the material and shape of the DUT before use. The pre-processor detects the change of the equivalent inductance of the coil in the probe caused by the position change of the DUT, and converts it into a corresponding voltage quantity. The voltage is proportional to the change in the distance between the measured surface and the probe, respectively.

The processor 5 includes a differential circuit, a summation circuit and a pulse counter, which are used to process the voltage signal output by the eddy current sensor and the corresponding preconditioner. The schematic diagram of the differential circuit is shown in Figure 8, and the schematic diagram of the summation circuit is shown in Figure 9 ;

The differential circuit eliminates the common mode interference signal. The source of the probe detection voltage deviation is mainly because the probe is not aligned with the axis of the rotor shaft, that is, the position eccentricity of the probe in the orthogonal direction; and for the two probes in opposite directions, the size of the eccentricity in the orthogonal direction is the same, so the impact on the output voltage of the probe when testing the displacement in this direction is the same (ie common mode interference). The differential circuit is used to collect the voltage signals representing the displacement in the direction collected by the two probes in the opposite direction. Subtraction can eliminate the influence of this part of the error, that is, common mode interference;

The floating position of the rotor is uncertain, and it cannot be guaranteed that the probe is always facing the outer normal direction of the rotor under test. However, due to the influence of the rotor eccentricity, the voltage signal measured by the probe and the displacement of the outer surface of the rotor in the forward direction of the probe are not directly facing the situation. The linear relationship below is affected by the eccentric size, and its bias situation is constantly changing, as shown in Figure 1, resulting in a large error when the eddy current sensor directly detects the radial displacement. For two probes in opposite directions, the displacements of the probe directions are opposite, and the eccentricity of the orthogonal direction is the same, so the voltage data collected by the two probes in the opposite direction can be subtracted to eliminate the orthogonality. The common mode interference of the direction eccentricity, and the sensitivity of the relative direction displacement detection is improved at the same time.

The summation circuit eliminates the influence of the rotor eccentricity on the high and low voltages of the speed detection, and ensures that the voltage signal representing the displacement in the detection direction remains unchanged when the rotor in the orthogonal direction of the detection direction is offset.

The summation circuit outputs the voltage to the pulse counter, and the pulse counter is used for speed measurement. According to the number (frequency) of high-voltage pulses per unit time, the current motor speed is obtained. If the number of high-voltage pulses measured per second is N, the rotational speed n=30Nrpm.

The application of the above-mentioned integrated detection system for the suspension position, deflection angle and rotational speed of the suspension motor rotor is as follows:

(1) Four eddy current sensors are arranged on the housing using a differential structure. The two opposite eddy current sensors are grouped together. The four eddy current sensors are spaced 90° apart in the plane. The installation directions of the probes 6 of the eddy current sensors all point to The center line of rotor 1 is used to detect the displacement of the outer surface of the radial test piece, and the displacement of the radial test piece is the rotor displacement;

(2) The radial test piece is installed and fixed on the rotor shaft by interference fit, so that the radial test piece rotates coaxially with the rotor;

(3) Start the suspension motor, and the probe of the eddy current sensor collects the information on the outer surface of the rotor in the opposite direction. During the rotation of the rotor, the shape of the outer surface of the rotor facing the probe changes continuously, and the probe collects a series of high and low voltage signals. Due to the constant change of the rotor displacement in the direction facing the probe, the displacement measured by the two probes in the opposite direction is also constantly changing, and there is no regularity (square waves with changing maximum and minimum values, even not square waves), resulting in inability to Directly perform rotational speed detection (as shown in Figure 1, when there is a certain and fixed displacement in the measured direction and the orthogonal direction, the change of the signals detected by the two eddy current sensors in the opposite direction). By adding the voltage signals collected by the eddy current sensors in opposite directions, a voltage signal with a constant amplitude can be obtained, and by frequency counting the voltage signal, the rotor speed can be obtained.

In step (1), the zero position adjustment is performed when the eddy current sensor is installed. After the suspension motor is processed and assembled, it is subjected to the gravity of the rotor during static state, and the rotor is not at the radial center position. The radial test piece is coaxial with the rotor. The eddy current sensor probe is calibrated in advance for the material and shape of the radial test piece to obtain the proportional coefficient between the displacement and the voltage of the radial test piece. The measurement is based on the voltage signal value obtained by the processor. The displacement in the direction of the eddy current sensor can be obtained, and the current floating position of the rotor can be determined according to the displacement;

Then adjust the initial position of the eddy current sensor probe through the thread, and complete the zero position adjustment when the output voltage of the propulsion device is zero. Complete the zero adjustment of the remaining eddy current sensors in the same way.

Example 2:

An application of an integrated detection system for the suspension position, deflection angle and rotational speed of a suspension motor rotor. The operation steps are as described in Embodiment 1. The difference is that the zero-adjustment tool 8 includes a hollow cylinder with one end open, and the bottom of the hollow cylinder is A cylinder is set at the center, as shown in Figure 4. The inner diameter of the hollow cylinder is the same as the outer diameter of the annular casing, which is used to fix the annular casing. The diameter of the cylinder is the same as the rotor shaft diameter, and the radial test piece is fixed during use. For the cylinder, the zero position adjustment of the eddy current sensor is carried out by adjusting the zero position of the tool to ensure that the center position of the radial test piece is the same as the center position during operation.

Example 3:

An application of an integrated detection system for the suspension position, deflection angle and rotational speed of a suspension motor rotor, the operation steps are as described in Embodiment 1, the difference is that in step (1), four eddy current sensors are respectively arranged at both ends of the rotor , determine the axial installation distance L of the eddy current sensors at both ends, and the ratio of the displacement difference (y1-y2) measured by the eddy current sensors in the corresponding direction at both ends to the axial installation distance (L), which is the deflection angle of the rotor in this direction The tangent value tanα of , the deflection angle α of the rotor can be obtained by calculating the arc tangent. The calculation principle diagram is shown in Figure 10.

Claims (9)

1. a suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system, it is characterized in that, comprise radial test piece, eddy current sensor, preconditioner and processor, wherein, eddy current sensor is connected with preconditioner , the preprocessor is connected with a processor, the preprocessor is used to supply power to the eddy current sensor and detect and analyze the circuit, and the processor is used to process the measurement results of the eddy current sensor; Four eddy current sensors are arranged on the suspension motor housings at both ends of the rotor using a differential structure respectively, two opposite eddy current sensors are a group, and the four eddy current sensors are spaced at 90° in-plane. The axial installation distance of the eddy current sensors at both ends is L, and the ratio of the displacement difference measured by the eddy current sensors in the corresponding directions at both ends to the axial installation distance is the tangent value tanα of the deflection angle of the rotor in this direction, and the arc tangent is calculated to obtain The deflection angle α of the rotor; The radial test piece includes an annular cylinder, the outer side of the annular cylinder is symmetrically convex with a circular arc segment, and the circular arc segment is concentric with the annular cylinder; The arc segment is provided with at least 2 segments; The processor includes a differential circuit, a summation circuit and a pulse counter, which are used to process the voltage signal output by the eddy current sensor and the corresponding preconditioner; Differential circuit eliminates common mode interference signal; The summation circuit eliminates the influence of the rotor eccentricity on the high and low voltages of the speed detection, and ensures that the voltage signal representing the displacement in the detection direction remains unchanged when the rotor in the orthogonal direction of the detection direction is offset; The summation circuit outputs the voltage to the pulse counter, and the pulse counter is used for speed measurement. According to the number of high-voltage pulses per unit time, the current motor speed is obtained. 2 . The integrated detection system for the suspension position, deflection angle and rotational speed of the suspension motor rotor according to claim 1 , wherein the radial test piece is made of conductor material and hard aviation aluminum. 3 . 3 . The integrated detection system for the suspended position, deflection angle and rotational speed of the suspension motor rotor according to claim 1 , wherein the eddy current sensor is selected as a high-frequency reflection type eddy current sensor. 4 . 4. The suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system as claimed in claim 1, wherein the eddy current sensor is fixed to the suspension motor casing through the annular casing, and the eddy current sensor is screwed through and connected to the suspension motor. Ring-shaped housing and suspension motor housing. 5. An application of the suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system as claimed in claim 4, wherein the operation steps are as follows: (1) Four eddy current sensors are arranged on the housing using a differential structure. The two opposite eddy current sensors are grouped together. The four eddy current sensors are spaced 90° apart in the plane. The probes of the eddy current sensors are installed in the direction of the rotor. The central axis is used to detect the displacement of the outer surface of the radial test piece, and the displacement of the radial test piece is the rotor displacement; (2) The radial test piece is installed and fixed on the rotor shaft by interference fit, so that the radial test piece rotates coaxially with the rotor; (3) Start the suspension motor, and the probe of the eddy current sensor collects the information on the outer surface of the rotor in the opposite direction. During the rotation of the rotor, the shape of the outer surface of the rotor facing the probe changes continuously, and the probe collects a series of high and low voltage signals. The voltage signals collected by the eddy current sensors in opposite directions can be added to obtain a voltage signal with a constant amplitude. The frequency of the voltage signal can be counted to obtain the rotor speed. 6. The application of the suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system as claimed in claim 5, characterized in that, in step (1), zero position adjustment is performed when the eddy current sensor is installed, and the suspension motor is processed and assembled After that, the rotor is not at the radial center position due to the gravity of the rotor when it is static, and the radial test piece is fixed by the zero-adjusting tool so that the radial test piece is coaxial with the rotor. The eddy current sensor probe is aimed at the material of the radial test piece in advance. The proportional coefficient between the displacement and the voltage of the radial test piece is obtained. During the measurement, the displacement in the direction of the eddy current sensor can be obtained according to the voltage signal value obtained by the processor, and the rotor can be determined according to the displacement. the current floating position; Then adjust the initial position of the eddy current sensor probe through the thread, and complete the zero position adjustment when the output voltage of the propulsion device is zero. 7. The application of the suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system as claimed in claim 6, wherein the zero-adjustment tooling comprises a hollow cylinder with an open end, and the center position of the bottom of the hollow cylinder is provided with Cylinder, the inner diameter of the hollow cylinder is the same as the outer diameter of the annular shell, and the diameter of the cylinder is the same as the rotor shaft diameter. 8. The application of the suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system as claimed in claim 6, wherein in step (1), 4 eddy current sensors are respectively arranged at both ends of the rotor to determine two eddy current sensors. The axial installation distance L of the end eddy current sensor, the ratio of the displacement difference measured by the eddy current sensor in the corresponding direction at both ends and the axial installation distance, is the tangent value tanα of the deflection angle of the rotor in this direction, and the arc tangent calculation can be obtained. The deflection angle α of the rotor. 9. The application of the suspension motor rotor suspension position, deflection angle and rotational speed integrated detection system as claimed in claim 8, is characterized in that, in step (3), for two probes of opposite directions, its probe direction displacement is opposite , and the eccentricity in the orthogonal direction is the same, so the displacement data collected by the two probes in the opposite direction can be subtracted to eliminate the common-mode interference of the eccentricity in the orthogonal direction.

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