JP2007085959A - Capacitive displacement sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a capacitive displacement sensor, capable of limiting the effects of stray capacitance to a minimum at the displacement measurement of the electrostatic capacitor. <P>SOLUTION: Between a pair of electrode pair comprising two sheet of electrodes, alternating voltage is impressed on a first electrode, and as a result of measuring the AC current generated on the second electrode, the distance between the two electrodes is obtained. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a capacitive displacement sensor, and more particularly to a capacitive displacement sensor suitable for use in a magnetic resonance force microscope (MRFM).

Of the two electrodes constituting the capacitor, if one electrode is the detection electrode and the other electrode is the electrode on the measured object side, the capacitance C ₀ generated between the two electrodes is expressed by the following equation (1). Given.

ここで、εは電極間の誘電率、Ｓは電極同士が対向する面積、ｄは電極間距離を表わす。つまり、静電容量Ｃ₀を計測することによって、検出電極の位置に対する被測定体の相対的な位置変位が得られる。静電容量式変位センサーの特徴は、電極同士が非接触で測長が行なえ、測長に際して被測定体に物理的な負荷を与えないことである。

Here, ε represents the dielectric constant between the electrodes, S represents the area where the electrodes face each other, and d represents the distance between the electrodes. That is, by measuring the capacitance C ₀ , the relative displacement of the measured object with respect to the position of the detection electrode can be obtained. The feature of the capacitive displacement sensor is that the electrodes can be measured without contact with each other, and a physical load is not applied to the measurement object during the measurement.

  FIG. 1 shows a configuration example of a commonly used capacitance type displacement sensor. In the figure, 1 is a detection electrode, and 2 is an object to be measured. The dielectric constant between the detection electrode 1 and the measured object 2 is ε, the distance is d, and the electrode area of the detection electrode 1 is S. Since the device under test 2 is a device under test and also serves as an electrode, the surface needs to be a good electrical conductor. Furthermore, it is necessary to set the surface potential to the same potential as the common ground of the detection circuit.

Reference numeral 3 denotes an electrode holder for fixing and holding the detection electrode 1. When the electrode holder 3 is a good electrical conductor, it is necessary to sandwich an insulating material between the electrode holder 3 and the detection electrode 1. Reference _numeral 4 denotes a reference standard capacitance whose capacitance is Cr. 5 is a wiring for electrically connecting the detection electrode 1 and the reference standard capacitance 4, and 6 is a wiring for transmitting an AC voltage signal to the reference standard capacitance 4. 8 is an AC constant voltage oscillator and 9 is an AC voltage signal receiver.

Here, assuming that the output voltage effective value of the AC constant voltage oscillator 8 is V ₀ , the effective voltage value V ₁ detected by the AC voltage signal receiver 9 is given by the following expression (2).

参照用標準静電容量Ｃ_rの容量値は既知であるため、実効電圧値Ｖ₁を計測することによって、検出電極１と被測定体２とが与える静電容量Ｃ₀の値を得ることができる。つまり、式（１）により、検出電極１の電極面積Ｓおよび検出電極１と被測定体２の間の誘電率εが既知であれば、得られたＣ₀の値から、検出電極１と被測定体２の距離ｄが導出される。

Since the capacitance value of the reference standard capacitance C _r is known, the value of the capacitance C _{0 provided} by the detection electrode 1 and the measured object 2 can be obtained by measuring the effective voltage value V _1. it can. In other words, if the electrode area S of the detection electrode 1 and the dielectric constant ε between the detection electrode 1 and the measured object 2 are known according to the equation (1), the detection electrode 1 and the coated electrode can be calculated from the obtained value of C _0. The distance d of the measuring body 2 is derived.

JP 2002-221402 A

In FIG. 1, the capacitance C ₀ between the electrodes 1 and 2 and the reference standard capacitance C _r are clearly shown. However, as a practical problem, stray capacitance as described below is generated in the capacitance detection circuit.
(1) When a conductor having the same potential as the ground is present in addition to the measurement object 2 around the detection electrode 1 and the holder 3, stray capacitance is generated between the conductor and the detection electrode 1.
(2) When a conductor having the same potential as the ground exists around the wiring 5, stray capacitance is generated between the conductor and the detection electrode 1.
(3) When a conductor having the same potential as that of the ground exists around the wiring 6, stray capacitance is generated between the conductor and the detection electrode 1.
When the total of the stray capacitances (1) to (3) is C _s , the effective voltage value V ₁ detected by the AC voltage signal receiver 9 is given by the following expression (3).

（３）式から、Ｃ₀に対してＣ_sが大きくなると（Ｃ₀＜Ｃ_s）、Ｖ₁の値はＣ_sに大きく依存することが理解できる。そして、電極間距離ｄの変位δｄに対するＶ₁の変化量δＶ₁は、下式（４）で与えられる。

From equation (3), it can be understood that when C _s increases with respect to C ₀ (C ₀ <C _s ), the value of V ₁ greatly depends on C _s . The change amount δV ₁ of V ₁ with respect to the displacement δd of the interelectrode distance d is given by the following equation (4).

（４）式から、Ｃ₀に対してＣ_sが大きくなると（Ｃ₀＜Ｃ_s）、δｄ／ｄに対するδＶ₁／Ｖ₁の比例係数Ｃ₀／（Ｃ₀＋Ｃ_r＋Ｃ_s）が減少し、ｄの変位検出感度が低下することが理解できる。

From equation (4), when C _s increases with respect to C ₀ (C ₀ <C _s ), the proportional coefficient C ₀ / (C ₀ + C _r + C _s ) of δV ₁ / V _{1 with} respect to δd / d decreases. It can be understood that the displacement detection sensitivity of d and d decreases.

For example, assume that a small detection electrode having a disk shape with a diameter of 1φ is used. Assuming that ε is a vacuum dielectric constant, when d = 10 μm, C ₀ = 0.70 pF is calculated. Further, when it is displaced to d = 20 μm, it is calculated as C ₀ = 0.35 pF. Further, assuming that the distance between the reference standard capacitance 4 and the AC constant voltage oscillator 8 is separated, a 1 m coaxial cable 1.5C-2V (characteristic impedance 75Ω, per unit meter) is used as the wiring 6. Assume that the capacitance is 67 pF). At this time, the stray capacitance C _s is 67 pF. Also, C _r = 5 pF and V ₀ = 1V.

In this case, when C ₀ = ₀ pF, V ₁ = 0.0694 V, when C ₀ = 0.70 pF, V ₁ = 0.0688 V, and when C ₀ = 0.35 pF, V ₁ = 0.0691 V. Become. That is, V ₁ is difficult to reflect the change in C ₀ and is determined by the value of the large stray capacitance C _s . The proportionality coefficient C ₀ / (C ₀ + C _r + C _s ) indicating the displacement detection sensitivity of the inter-electrode distance d is 0.0688, which is significantly lower than 1.

  Thus, in the prior art, when a capacitance sensor that can generate only a small capacitance, such as a small capacitance displacement sensor, is handled, the stray capacitance greatly affects.

  As the stray capacitance increases, the following problems arise.

1. The output voltage V ₁ largely depends on the stray capacitance C _s , not the target C ₀ . Specifically, when the distribution cable is changed, such as bending the distribution cable during measurement, the stray capacitance generated in the distribution cable changes, causing an error in the amount of change in the position of the measured object. To do.

2. The dynamic range of the output voltage V ₁ with respect to the change of C ₀ cannot be obtained efficiently. That is, V ₁ has a finite value determined by a large stray capacitance C _s , and if the voltage range is matched with that value, a small change amount of V ₁ with respect to the change of C ₀ cannot be measured efficiently.

3. To relative displacement .delta.d / d, the voltage measurement accuracy .DELTA.V ₁ / V, depending on the reciprocal of the stray capacitance C _s, the presence of a large stray capacitance C _s, the voltage measurement accuracy drops remarkably.

  In addition, the following problems arise when the stray capacitance is reduced.

  4). Restrictions are imposed such that the wiring cable cannot be lengthened or the oscillator must be incorporated in the measurement system. This limitation becomes a problem in an environment where the measurement system is limited to a narrow space, or in a special environment such as a vacuum, a low temperature, a high temperature, or a high magnetic field.

  An object of the present invention is to provide a capacitance type displacement sensor that can minimize the influence of stray capacitance when measuring the displacement of the capacitance in view of the above points.

In order to achieve this object, a capacitive displacement sensor according to the present invention includes:
Of the two electrode pairs that generate capacitance, an AC constant voltage is applied to the first electrode, and as a result, the AC current generated in the second electrode is measured, thereby the two electrodes It is characterized in that the distance between electrodes is obtained.

  Further, the AC current proportional to the capacitance is converted into a voltage value, and then output as an amplitude value of the AC voltage by a phase detection circuit, whereby the distance between the two electrodes is obtained. Yes.

  In addition, the electrode pairs are provided one by one in the direction of the three axes, and one of the electrode pairs is constituted by one common electrode.

  Further, the frequency of the AC constant voltage applied to each of the electrode pairs is different from each other.

According to the capacitive displacement sensor of the present invention,
Of the two electrode pairs that generate capacitance, an AC constant voltage is applied to the first electrode, and as a result, the AC current generated in the second electrode is measured, thereby the two electrodes Since the distance between the electrodes was calculated,
It is possible to provide a capacitance type displacement sensor that can minimize the influence of stray capacitance when measuring the displacement of the capacitance.

  Embodiments of the present invention will be described below with reference to the drawings.

  2 and 3 show an embodiment of the capacitance sensor according to the present invention. In the present embodiment, an AC constant voltage is applied to the electrode 16 provided on the measurement object 15 to detect an AC minute current generated in the detection electrode 11, and a capacitance generated between the electrode 11 and the electrode 16 is measured. By doing so, the relative distance d from the detection electrode 11 to the measured object 15 is obtained. Specific examples of each component will be described below.

  11: Detection electrode. This is an electrode for detecting a minute current flowing from the electrode 16. It is surrounded by an insulating spacer 12 and further fixed by a detection electrode holder 13. The detection electrode 11 is connected to the inner conductor of the coaxial cable 14, and the generated alternating current signal is transmitted to the current-voltage converter 19.

  12: Insulating spacer. The detection electrode 11 and the detection electrode holder 13 play a role of mediating so as not to make electrical contact.

  13: Detection electrode holder. This is a conductive holder that fixes and supports the detection electrode 11 and the insulating spacer 12. Further, it also serves as an electric shield so that an external noise electric field is not transmitted to the detection electrode 11. For this reason, it is necessary to make electrical contact with the outer conductor of the coaxial cable 14 so that the potential is the same as that of the ground. Further, in order to measure the relative position displacement of the measurement object with high sensitivity, it is necessary to approach the detection electrode 11 to the vicinity of the electrode 18, and it is necessary to provide a coarse movement mechanism that can approach this holder. Further, when there is a possibility of contact between the detection electrode 11 and the electrode 18 when approaching, in order to avoid this contact, the electrode surface of the detection electrode 11 is as far back as possible from the detection electrode holder surface. Need to design.

  14: Coaxial cable. It plays a role of transmitting an alternating current signal generated at the detection electrode 11 to the current-voltage converter 19. Further, it also serves to shield the external noise electric field from entering the alternating current signal and to shield the outer conductor from the stray capacitance between the inner conductor and the inner conductor of the coaxial cable 18. If these roles can be played, there is no problem even with a general shielded cable. Further, it is preferable that the characteristic impedance of the coaxial cable be as large as possible.

  15: Measurement object. It is a measurement object whose relative position is measured by the detection electrode 11. The electrical characteristics such as electrical conductivity are not particularly limited, but are limited to those in which the insulating spacer 17, the electrode 16, and the wiring 18 can be installed on the surface.

  16: Electrode. It is an electrode deposited or attached to the measurement object. The AC constant voltage generator 22 applies an AC constant voltage between the electrode and the ground. An insulating spacer 17 is sandwiched between the measurement object 15 and electrical contact with the measurement object is interrupted. For example, the copper foil is cut into a length of 12 mm, a width of 8 mm, and a thickness of 0.3 mm, and the electrode surface is finished in a mirror-like shape in parallel, and is attached to the insulating spacer 17 with an adhesive.

  17: Insulating spacer. An insulating film attached to the measurement object 15. It plays the role of breaking the electrical contact between the measurement object 15 and the electrode 16. For example, a commercially available mylar tape is used. At this time, the adhesive surface of the tape is adhered to the object to be measured 15, and an epoxy resin adhesive is thinly applied to the surface opposite to the adhesive surface, and the electrode 16 is adhered.

  18: Coaxial cable. It plays a role of transmitting an AC voltage signal from the AC constant voltage oscillator 22 to the electrode 16. In addition, it also serves to shield the external noise electric field from entering the alternating current signal, and to shield electrostatically by the outer conductor so that no stray capacitance is generated between the inner conductor and the inner conductor of the coaxial cable 18. . As long as these roles can be fulfilled, there is no problem even with a general shielded cable. Care should be taken that the bonding between the inner conductor and the electrode 18 does not hinder the approach operation of the capacitive sensor due to physical unevenness formed by the contact. For this purpose, the electrode 18 is secured widely and terminal attachment is performed at the edge of the electrode 18. It is sufficient for the outer conductor to have the same potential as that of the ground, and it is not always necessary to make contact with the measured object 15.

  19: Current-voltage converter. An alternating current signal generated from the detection electrode 11 is detected, and the detected current value is converted into a voltage value. The converted AC voltage signal is sent through the buffer amplifier 20 to the detection circuit 21 that outputs the amplitude value of the AC voltage. A small value is required for the internal resistance (input impedance) of the current-voltage converter 19. Further, in order to ensure the detection accuracy δI of the minute current, the input bias current value needs to be sufficiently larger than δI.

  20: Buffer amplifier. The minute input voltage signal is amplified to a voltage level that can be detected by the detection circuit 21.

  21: Detection circuit. This is a detection circuit that outputs the amplitude value of the input AC voltage. The detection circuit 21 shown in FIG. 2 means a detection circuit composed of a rectifier circuit and a smoothing circuit. In the embodiment of the circuit shown by the broken line frame 21 in FIG. 3, the detection circuit 21 is demultiplexed from the output of the AC constant voltage oscillator 22. This means a phase detection circuit 24 that performs phase detection using a signal as a reference wave. In the latter case, a low-frequency amplifier 25 for cutting off harmonics is provided in the subsequent stage.

  22: AC constant voltage oscillator. An alternating constant voltage signal is oscillated and transmitted to the electrode 16.

  23: Recorder. Display and record the voltage value of the DC voltage signal.

  24: Phase detection circuit. A phase detection circuit that outputs the amplitude value of the input AC voltage. A signal demultiplexed from the output of the AC constant voltage oscillator 22 is used as a reference wave.

  25: Low frequency amplifier. A high frequency is cut in the output signal of the phase detection circuit.

  In FIG. 2, the device arranged inside the dotted line is placed in the same measurement environment as the object 15 to be measured, for example, under vacuum, at low temperature and high temperature, or under a strong magnetic field. The device arranged outside the dotted line is not particularly required to be placed in the same measurement environment as the measured object 15, and the coaxial cables 14, 18 may be lengthened and installed in a room temperature environment.

  Further, depending on the situation, when the detection electrode 11 is approached to the electrode 16, there may be a case where both come into contact with each other. Since there is a possibility that a large current from the AC constant voltage oscillator 22 flows directly into the current-voltage converter 19, an overcurrent protection circuit is separately required before the current-voltage converter 19.

  Further, as a modification of FIG. 2, a method of reading an electric current generated in the electrode 16 by applying an AC constant voltage to the capacitance sensor side is also conceivable.

  Now, the capacitance sensor as described above operates as follows.

  1. The detection electrode 11 is approached in the vicinity of the electrode 16 in advance. For the approach, a coarse movement mechanism provided in the detection electrode holder 13 is used. Further, when there is a possibility that the electrodes come into contact with each other in the approach work, it is necessary to turn off the output of the AC constant voltage oscillator 22. Further, after the approach, before the output of the AC constant voltage oscillator 22 is turned ON, the electrical contact confirmation work between the outer conductor of the coaxial cable 14 and the electrode 16 and between the inner conductor of the coaxial cable 14 and the electrode 16 is performed. And the detection electrode holder 13 and the electrodes are not physically in contact with each other.

2. The output of the AC constant voltage oscillator 22 is turned ON, and an AC constant voltage is applied to the electrode 16. Assume that the angular frequency and effective amplitude of the output AC voltage are ω and V ₀ , respectively.

  3. The effective value I of the minute alternating current generated in the detection electrode 11 is detected.

4). The relative displacement d of the measured object is obtained from the obtained effective current value I by the following procedure. The capacitance generated between the electrodes 11 and 16 is C ₀ , and the internal resistance of the current-voltage converter 19 is R _IV . Further, the total of the stray capacitance generated in the capacitance type sensor, the stray capacitance generated in the coaxial cables 14 and 18, and the stray capacitance generated between the electrode 16 and the measured object 15 is defined as C _s . At this time, the current value I detected by the current-voltage converter 19 is given by the following equation (5).

ここで、ωＲ_IVＣ_s《１が成立すれば、Ｉ＝Ｃ₀Ｖ₀と近似され、電流値ＩはＣ_sに依存しなくなる。よって、Ｖ₀は既知であるので、電流値ＩからＣ₀を導出することができる。つまり上式（５）より、Ｓおよびεが既知であれば、得られたＣ₀から被測定体１５の相対距離ｄが換算される。また、電極間距離ｄの変位δｄに対するＶ₁の変化量δＶ₁は、次式（６）で与えられる。

Here, if ωR _IV C _s << 1 holds, it is approximated as I = C ₀ V _0, and the current value I does not depend on C _s . Therefore, since V ₀ is known, C ₀ can be derived from the current value I. That is, from the above equation (5), if S and ε are known, the relative distance d of the measured object 15 is converted from the obtained C ₀ . Further, the change amount δV ₁ of V ₁ with respect to the displacement δd of the inter-electrode distance d is given by the following equation (6).

検出値相対精度δＩ／Ｉのδｄ／ｄに対する比例係数は−１であり、浮遊容量Ｃ_sには依存しない。

The proportional coefficient of the detected value relative accuracy δI / I with respect to δd / d is −1 and does not depend on the stray capacitance C _s .

A standard implementation example is given for the condition of ωR _IV C _s << 1. The impedance 1 / ωC _s generated by the stray capacitance is 1.6 MΩ when ω = 2π × 1000 Hz and C _s = 100 pF. If a material having an R _IV of about 10Ω is selected, ωR _IV C _s = 6.3 × 10 ⁻⁶ << 1 can be satisfied.

FIG. 4 is an example of the measurement result. The diameter of the detection electrode 11 is 3.8φ. The measurement object 15 was a piezo drive stage. Area of the electrode 16 affixed to the stage is 96 mm ^2. The input impedance of the current-voltage converter 19 is 10Ω. About the phase detection of the alternating voltage obtained from the current-voltage converter, the time constant was set to 0.16 s. The output constant voltage effective value of the AC constant voltage oscillator 12 was 7.1 V _rms and the frequency was 1103 Hz.

The horizontal axis in FIG. 4 indicates the scanning distance P _os when the electrodes are brought close to an interval of about 50 μm in advance, the scanning distance P _os is 0 μm, and the measured object is scanned 100 μm in the direction in which the electrodes are separated. The horizontal axis position measurement indicates a measurement result using a strain gauge sensor provided on the stage. The vertical axis represents the current value received by the current-voltage converter.

The position measurement accuracy was approximately δP _os = 0.008 μm, 0.05 μm, and 0.11 μm at P _os = 0.0 μm, 50 μm, and 100 μm.

5 and 6 show an embodiment of a three-axis capacitive displacement sensor according to the present invention. FIG. 5 shows parts included in the measurement system. 5A shows a state in which a common detection electrode 37 is attached to the measurement object 36, and three electrodes 31, 31 ^′ , 31 ^″ perpendicular to each other are installed in the vicinity of the electrode. 5A shows a view in which only the measured object 36 is taken out in Fig. 5A, and a sectional view of the detection electrode 37 attached to the measured object 36 is shown in Fig. 5E. 5C shows a state in which the device to be measured 36 is taken out in FIG.5A, where the back side of the electrode surface is illustrated in FIG.5A and the electrode surface side is illustrated in FIG.5C. Fig. 5 (d) shows a cross-sectional view of the electrode, and Fig. 6 shows, as a block diagram, constituent elements mainly required for capacitance measurement. Indicates the part included in the described measurement system, the prime being the different axis in the capacitive sensor and detector The difference in structure and function is not indicated by the presence or absence of prime, and specific examples of each component will be described below.

  31: Electrode. The triaxial electrode is composed of three electrodes orthogonal to each other. Each electrode is attached to an electrically conductive holder 33 with an insulating spacer 32 in between. These electrodes are in electrical contact with the inner conductor of the coaxial cable 34, and AC voltages having different frequencies are applied from the three AC constant voltage oscillators 42, respectively. Further, the electrode surface is placed behind the surface of the electrically conductive holder 33 and the insulating spacer 32 so that the electrode surface does not come into electrical contact with the common electrode 37.

  32: Insulating spacer. The electrode 31 and the electrode holder 33 are sandwiched so as not to be in electrical contact.

  33: Electrode holder. The electrode 31 and the insulating spacer 32 are fixed. Further, it is brought into electrical contact with the outer conductor of the coaxial cable 34 so that the potential is the same as that of the ground.

  34: Coaxial cable. A coaxial cable for electrically connecting the electrode 31 and the AC constant voltage oscillator 42. The outer conductor of the coaxial cable 34 is provided so that constant voltage signals for the other shafts are not mixed with each other and no electrostatic capacitance is generated between the coaxial cable 39 and the coaxial cable 39. The potential of the outer conductor is made common with the ground of the AC constant voltage oscillator 42.

  35: Arm. A support plate on which triaxial electrodes are installed so as to be perpendicular to each other. In order to measure the relative position displacement of the measured object 36 with high sensitivity, it is necessary to approach the electrode 31 in the vicinity of the common detection electrode 37, and this arm needs to have a coarse motion mechanism that can be approached. .

  36: Measurement object. A measurement object whose relative position is measured. The electrical characteristics such as electrical conductivity are not particularly limited, but are limited to those in which the insulating spacer 32, the electrode 37, and the wiring 39 can be installed on the surface. Further, it is possible to mount a measurement object in which a part of three surfaces orthogonal to each other are cut out or an object having three surfaces orthogonal to each other so that three-axis position displacement measurements orthogonal to each other are possible. Limited to measured object.

  37: Common detection electrode. An electrode having a surface parallel to each surface of the triaxial electrode 31. An electrostatic capacity corresponding to the distance in each axial direction is generated between the electrode and the electrode 31. A minute current proportional to the capacitance is generated by an alternating voltage applied to the electrode 31 with different axial frequencies. The generated three types of alternating current are sent to the current-voltage converter 40 through the same coaxial cable 39.

  38: Insulating spacer. The common detection electrode 37 and the measured object 36 are sandwiched so as not to be in electrical contact.

  39: Coaxial cable. Occurring at the common detection electrode 37 while shielding the external noise electric field from being mixed with the alternating current signal and shielding the outer conductor so that stray capacitance does not occur between the inner conductor and the inner conductor of the coaxial cable 34. It plays a role of transmitting the minute alternating current signal thus transmitted to the current-voltage converter 39.

40: Current-voltage converter. An alternating current signal generated from the common detection electrode 37 is detected, and the detected current value is converted into a voltage value. The converted AC voltage signal is sent through the buffer amplifier 41 to the detection circuit that outputs the amplitude value of the AC voltage in the phase detection circuit 43. When the internal resistance of the current-voltage converter 40 (input impedance) and R _IV, it is necessary to select a device having a small R _IV. Further, in order to ensure the detection accuracy δI of the minute current, the input bias current value needs to be sufficiently larger than δI.

  41: Buffer amplifier. The minute input voltage signal is amplified to a voltage level that can be detected by the phase detection circuit 43.

  42: AC constant voltage oscillator. An AC constant voltage signal is oscillated and transmitted to the electrode 41.

  43: Phase detection circuit. A phase detection circuit that outputs the amplitude value of the input AC voltage. A signal demultiplexed from the output of the AC constant voltage oscillator 42 is used as a reference wave.

  44: Low frequency amplifier. A high frequency is cut in the output signal of the phase detection circuit.

  45: Recorder. Display and record the voltage value of the DC voltage signal.

  Further, as a modification of FIG. 5, a method in which a common detection electrode is provided on the arm 35 side and an AC constant voltage is applied to the triaxial electrode provided on the measured object 36 side is also conceivable.

  The three-axis capacitance sensor as described above operates as follows.

  1. The triaxial electrode 31 is approached in the vicinity of the common detection electrode 37 in advance. For the approach, a coarse movement mechanism provided in the arm 35 is used. Further, when there is a possibility that the electrodes come into contact with each other in the approach work, it is necessary to turn off the output of the AC constant voltage oscillator 42. After the approach, before the output of the AC constant voltage oscillator 42 is turned ON, the electrical contact confirmation work between the outer conductor of the coaxial cable 34 and the common detection electrode 37 and between the inner conductor of the coaxial cable 34 and the common detection electrode 37 is performed. Then, it is confirmed that the common detection electrode 37 and the detection electrode holder 33, and the common detection electrode 37 and the triaxial electrode 31 are not in physical contact.

2. The output of the AC constant voltage oscillator 42 is turned ON, and an AC constant voltage is applied to the triaxial electrode 31. Assume that the angular frequency and effective amplitude of the output AC voltage are ω and V ₀ , respectively. For AC signals for three axes, the frequencies used for each axis should not be mixed with each other, such as ω ₀ , αω ₀ , βω ₀ (where α and β are irrational numbers).

3. The effective values I, I ^′ , I ^″ of the triaxial minute alternating current generated in the common detection electrode 37 are detected.

4). The relative displacement d of the measured object is obtained from the obtained effective current value I by the following procedure. Similarly, d ^′ and d ^″ are obtained from I ^′ and I ^″ . The capacitance generated between the electrodes 31 and 37 is C ₀ , and the internal resistance of the current-voltage converter 40 is R _IV . Further, the total of the stray capacitance generated in the capacitance type sensor, the stray capacitance generated in the coaxial cables 34 and 39, and the stray capacitance generated between the common detection electrode 37 and the measured object 36 is defined as C _s . . At this time, the current value I detected by the current-voltage converter 40 is given by the following equation (7).

ここで、ωＲ_IVＣ_s《１が成立すれば、Ｉ＝Ｃ₀Ｖ₀と近似され、電流値ＩはＣ_sに依存しなくなる。よって、Ｖ₀は既知であるので、電流値ＩからＣ₀を導出することができる。つまり上式（７）より、Ｓおよびεが既知であれば、得られたＣ₀から被測定体３６の相対距離ｄが換算される。また、電極間距離ｄの変位δｄに対するＶ₁の変化量δＶ₁は、次式（８）で与えられる。

Here, if ωR _IV C _s << 1 holds, it is approximated as I = C ₀ V _0, and the current value I does not depend on C _s . Therefore, since V ₀ is known, C ₀ can be derived from the current value I. That is, from the above equation (7), if S and ε are known, the relative distance d of the measured object 36 is converted from the obtained C ₀ . Further, the change amount δV ₁ of V ₁ with respect to the displacement δd of the interelectrode distance d is given by the following equation (8).

検出値相対精度δＩ／Ｉのδｄ／ｄに対する比例係数は−１であり、浮遊容量Ｃ_sには依存しない。

The proportional coefficient of the detected value relative accuracy δI / I with respect to δd / d is −1 and does not depend on the stray capacitance C _s .

A standard implementation example is given for the condition of ωR _IV C _s << 1. The impedance 1 / ωC _s generated by the stray capacitance is 1.6 MΩ when ω = 2π × 1000 Hz and C _s = 100 pF. If a material having an R _IV of about 10Ω is selected, ωR _IV C _s = 6.3 × 10 ⁻⁶ << 1 can be satisfied.

  Widely applicable to MRFM equipment.

It is a figure which shows the conventional electrostatic capacitance type displacement sensor. It is a figure which shows one Example of the electrostatic capacitance type displacement sensor concerning this invention. It is a figure which shows one Example of the electrostatic capacitance type displacement sensor concerning this invention. It is a figure which shows an example of the measurement result by the electrostatic capacitance type displacement sensor of this invention. It is a figure which shows another Example of the electrostatic capacitance type displacement sensor concerning this invention. It is a figure which shows another Example of the electrostatic capacitance type displacement sensor concerning this invention.

Explanation of symbols

1: detection electrode, 2: object to be measured, 3: electrode holder, 4: standard capacitance for reference, 5: wiring, 6: wiring, 8: AC constant voltage oscillator, 9: AC voltage signal receiver, 11: Detection electrode, 12: insulation spacer, 13: detection electrode holder, 14: coaxial cable, 15: measured object, 16: electrode, 17: insulation spacer, 18: coaxial cable, 19: current-voltage converter, 20: buffer amplifier , 21: detector circuit, 22: AC constant voltage oscillator, 23: recorder, 24: phase detector, 25: low frequency amplifier, 31: electrode, 32: insulating spacer, 33: electrode holder, 34: coaxial cable, 35 : Arm, 36: measurement object, 37: common detection electrode, 38: insulating spacer, 39: coaxial cable, 40: current-voltage converter, 41: buffer amplifier, 42: AC constant voltage oscillator, 43 Phase detector, 44: low-frequency amplifier, 45: recorder

Claims (4)

Of the two electrode pairs that generate capacitance, an AC constant voltage is applied to the first electrode, and as a result, the AC current generated in the second electrode is measured, thereby the two electrodes A capacitance type displacement sensor characterized in that a distance between electrodes is obtained. The AC current proportional to the capacitance is converted into a voltage value and then output as an AC voltage amplitude value by a phase detection circuit, whereby the distance between the two electrodes is obtained. Item 14. A capacitance type displacement sensor according to Item 1. 3. The capacitive displacement sensor according to claim 1, wherein the electrode pairs are provided in pairs in three axial directions, one of which is composed of one common electrode. 4. The capacitive displacement sensor according to claim 3, wherein the frequency of the AC constant voltage applied to each electrode pair is a different frequency.

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