JP2002206948A - Displacement sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a displacement sensor being tough enough against environmental changes and capable of measuring an absolute value of displacement within a range of several mms, at a reproduction resolution of about several nms even in air, and its measuring method. SOLUTION: The displacement sensor includes a resonance circuit consisting of a capacitor and a coil, with at least one of the capacitor and the coil being varied correspondingly to displacement. If a phase change from the resonance circuit which occurs as a resonance frequency is shifted by application of an alternating current to the resonance circuit exceeds a predetermined range, the application frequency is increased or decreased by a predetermined value to keep the phase change within the predetermined range. The amount by which the frequency is increased or decreased correspondingly to the displacement and the amount of phase shift at the latest application frequency are measured and the displacement is determined from these measurements.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement sensor for measuring an absolute value of a minute displacement with high accuracy, and more particularly, to a displacement sensor for minutely measuring the absolute value of a displacement in a measuring range of several mm with a resolution of several nm. About.

[0002]

2. Description of the Related Art Heretofore, as a displacement sensor for measuring an absolute value of a displacement with a resolution of about several nm, for example, a capacitance sensor that is robust against environmental fluctuations and has high reproducibility of measured values even in the air has been preferred. Although known, its measurement range is a minute range of about 0.05 mm, and its use has been greatly limited.

In the displacement measurement using a laser interference system in the air, the measurement range can be sufficiently large, on the order of several thousand mm, but the displacement is mainly due to the fluctuation of the refractive index of the air in the optical path due to the fluctuation of the air. Without this, the optical path length fluctuates, the expectation of the accuracy of the measured value is low, and the resolution considering the reproducibility is 1
It stays at about 00 nm. Moreover, the measurement by the laser interferometer is a difference value measurement that must be counted separately for an integer multiple of the wavelength, and has a problem that it is not a direct absolute value measurement.

[0004]

That is, in this laser interferometer measurement, it is necessary to form a variable-length vacuum optical path corresponding to the displacement in a vacuum in order to achieve both the expansion of the measurement range and the refinement of the reproduction resolution. However, the implementation means is complicated and large-scale as compared with the measurement range, and as a result, it is large and expensive.

An object of the present invention is to make the absolute value of the displacement several millimeters in the air with a reproducibility of about several nm even in the air, in view of the current situation of the conventional precision displacement sensor described above, which is robust to environmental fluctuations.
An object of the present invention is to provide a displacement sensor capable of measuring in a range of m and a measuring method thereof.

[0006]

In order to achieve the above object, the present invention provides a conventional dynamic circuit using a frequency applied to a variable length resonance circuit corresponding to displacement and a phase signal obtained from the resonance circuit. An object of the present invention is to provide an absolute value displacement sensor in which the range (measurement range / reproduction resolution) is greatly expanded from about 10 ^{4 to} about 10 ⁶ . In summary of the present invention, including a resonance circuit consisting of a capacitor and a coil,
At least one of the displacement sensors is configured to change in response to the displacement, and when an AC current is applied to the resonance circuit, a phase change from the resonance circuit that occurs according to a shift in the resonance frequency exceeds a predetermined range. The applied frequency is adjusted by a predetermined value to keep the phase change within the predetermined range, and the phase shift amount at the latest applied frequency and the amount of the frequency corresponding to the displacement are measured,
The measurement method of the displacement sensor for obtaining the displacement from these measured values is provided.

In the following description of a preferred embodiment of the present invention, 1) the capacitance of the capacitor is formed directly or via a relay electrode depending on the facing area between two electrodes, and 2) The inductance of the coil is formed in accordance with the position between the coil and the magnetic material entering and exiting the coil, and the position is changed in accordance with the displacement. 3) The phase of the detection signal of the resonance circuit is a displacement sensor whose phase difference is determined based on the phase of the applied frequency.4) The phase difference is determined by digital phase detection means. 5) a displacement sensor in which the adjustment value of the applied frequency is an integral multiple of a predetermined value, 6) the applied AC waveform is a rectangular having a constant amplitude. 7) One cycle of the rectangular wave is constituted by a plurality of rectangular waves having different time widths, and harmonics of a predetermined lower order such as three times or five times the fundamental wave are removed. Displacement sensor, 8) the predetermined value is a displacement sensor corresponding to the phase change range in claim 5, 9) the applied frequency and the detection value of the discrete phase in claim 4 are in correspondence with the displacement. A displacement sensor linearized using the integer value as a parameter according to claim 6 will be described.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram for explaining the capacitance between two opposing metal plates. Assuming that lines of electric force are generated only in the vertical direction on the metal plate, the distance between the two electrodes shown in FIG. The capacitance C ₁ is

(Equation 1) It can be expressed as. Here, ε ₀ is the dielectric constant of the substance inserted between the electrodes, S is the facing area, and d
₁ is a gap amount between the electrodes. Here, consider the case where the two electrodes move in the up and down direction while keeping the gap constant, and consider the facing area S as

(Equation 2) Then the capacitance C ₁ is

(Equation 3) ... Equation (1) is obtained. Here, χ is the displacement in the moving direction, and b is the width of the electrode, but it can be seen that the capacitance changes in proportion to the displacement χ.

Considering the capacitance between two metal cylinders arranged coaxially as shown in FIG. 2, it is assumed that lines of electric force are generated only in the direction perpendicular to the cylinder surface, as in FIG. Approximately,

(Equation 4) ... Equation (2) It can be seen that the capacitance also changes in proportion to the relative displacement in the axial direction.

As shown in FIG. 3, even when a metal plate to be measured is used as an intermediate electrode, a metal plate 1 and a metal plate 2 which are arranged on the same plane with an insulating plate interposed therebetween.
Also changes in proportion to the displacement χ as before. That is, in the displacement sensor, it is important how to extract an electric signal from the above-mentioned capacitance and accurately associate the electric signal with the displacement χ.

Next, the detection principle of the present invention will be described with reference to FIG. FIG. 4 shows that a coil of inductance L, a capacitor of capacitance C, and a resistor of R are connected in series,
This is a circuit to which a voltage E is applied to both ends. When the current flowing through this circuit is i,

(Equation 5) ... Expression (3) holds. Here, q is a charge amount, and

(Equation 6) ... Equation (4) or

(Equation 7) Equation (3) becomes the following equation.

(Equation 8) ... Equation (5)

Here,

(Equation 9) ... Equation (6) and Laplace transform gives

(Equation 10) ... Equation (7), and the Laplace operator s is

[Equation 11] By calculating the phase ψ of the charge amount q with respect to the input voltage A,

(Equation 12) Expression (8) is obtained.

This phase characteristic is, as shown in FIG.

(Equation 13) When

[Equation 14] And an S-shaped curve passing through.

Therefore,

(Equation 15) ... In equation (9), the origin is

(Equation 16) And consider the state of change near the origin.

That is,

[Equation 17] Then, equation (8) becomes

(Equation 18) ... (10) (ie, linear approximation) can be obtained.

Further, from equations (7) and (9),

[Equation 19] ... Equation (11), and

(Equation 20) Is added to the equation (10),

(Equation 21) Expression (12) is obtained.

Here, assuming that L and ω are constant, the phase (−
Consider how Δ を) varies with capacitance C. As described with reference to FIG. 1 to FIG.
When there is a proportional relationship with, consider a small displacement Δχ from a certain reference position χ ₀ ,

(Equation 22) By substituting into equation (12) and rearranging, the following equation is obtained.

(Equation 23) ... Expression (13)

Here,

(Equation 24) ... The frequency ω is selected so as to obtain Expression (14). At this time, equation (1)
3) is

(Equation 25) Expression (15) shows that the phase changes in proportion to the displacement.

Next, from equations (14) and (15), χ ₀
If you delete

(Equation 26) ... Expression (16) can also be expressed. FIG. 6 illustrates equation (16). The applied frequency ω corresponding to the reference position χ ₀ (formula (1)
It can be understood that the phase changes in proportion to the displacement from the reference position if the driving is performed in (4)). The gradient is

[Equation 27] It is. Therefore, if the phase change (−Δψ) is electrically and accurately detected, the displacement χ from the reference position can be accurately known. Since the electrical phase difference can be accurately detected by an existing detection technique including digital phase detection means, the description is omitted here. The above is the basic detection principle of the present invention.

Now, as can be understood from FIG. 6, what can be approximated by a straight line is

[Equation 28] In Expression (14), Δχ << 1, that is, a case where the reference position is sufficiently close to 近 傍₀ .

When the distance is far from the reference position, the displacement is obtained from the phase according to the following procedure. That is, as shown in FIG. 7, a plurality of reference positions are set, and an applied frequency corresponding to the position is determined.

FIG. 7 shows the reference positions as χ ₀₁ , χ ₀₂ , χ ₀₃ ,.
This is an example of setting At the reference position χ ₀₁ , the applied frequency is calculated according to equation (14).

(Equation 29) Similarly, at the reference position χ ₀₂ ,

[Equation 30] At reference position χ ₀₃ ,

(Equation 31) In this case, linear approximations can be made in the vicinity of each reference point.

Here, the connection method when the displacement gradually increases will be described. Since each straight line is an approximate straight line near the reference position, the range in which the straight line can be approximated in advance is (−Δψ).
_min ) and (−Δψ _max ) are set in advance. Now, when excited at applied frequency omega _1, the characteristic is located on a straight line (1), it is assumed that exceeds the upper limit (-Derutapusai _max), it became phase (-Derutapusai _1). In this case To change the applied frequency ω _2. Then, the phase changes to (−Δψ ₂ ) despite the same displacement.

[0024] If the displacement is further increased, according to the characteristics of the straight line (2), may be determined displacements from the phase may be changed and the applied frequency according to the previous steps to omega ₃ If further exceeding the limit. It is easy to expand the measurement range by following this procedure.

On the contrary, when the displacement becomes small, when the phase on the straight line (2) exceeds (−Δψ _min ) and becomes small,
The applied frequency ω ₂ may be changed to the applied frequency ω _1. The displacement at which the phase becomes (-Δψ _max ) on the straight line (1) is (χ ₀₁ )
_max, and the displacement at which the phase becomes (−Δψ _min ) on the straight line (2) is (χ ₀₂ ) _min , where (χ ₀₁ ) _max > (χ ₀₂ ) _min
FIG. 7 shows the case (1).

Next, consider the case of (χ ₀₁ ) _max = (χ ₀₂ ) _min . That is, as shown in FIG. 8, when the displacement is the upper limit (−Δψ _max ) on the straight line (1), the reference positions { ₀₁ , χ} are lower than the lower limit (−Δψ _min ) on the straight line (2). _The case where ₀₂ is set is considered, but since the interval between the set positions can be set to be the largest, there is an advantage that the number of reference positions can be minimized if the measurement range of the displacement χ is fixed. However, it is necessary to accurately set the reference position or the applied frequency by calculation or experiment.

If it is difficult to strictly set the applied frequency to a predetermined value, the following may be performed. That is, if a certain reference frequency ω _b is determined and an integer multiple thereof is used as the applied frequency, the reference position in this case is given by Expression (14).

(Equation 32) Therefore, the displacement may be obtained from the phase detected using this value. In this case, as shown in FIG. 9, for a displacement, always as one or more approximate straight line is present, it is necessary to determine a reference frequency omega _b so that there is a region of overlap.

In the above description, the detected phase is the phase difference between the reference signal and the signal representing the amount of charge flowing through the circuit. However, the voltage v generated in the resistor R is simply obtained by the circuit shown in FIG.

[Equation 33] Therefore, if equation (4) is substituted into this,

(Equation 34) By electrically integrating the generated voltage v with time, the charge amount q can be easily obtained.

Further, while the mathematical description so far has been described in terms of the relationship between the drive voltage E and the charge amount q, the drive voltage E and the voltage v
The same result can be obtained by describing the relationship with Further, the present invention is not limited to the circuit shown in FIG. 4, and various modifications can be considered in the present invention. For example, in the case of FIG. 10, it is a practical method to form the inductance in FIG. 4 with a transformer, apply a drive voltage there, and detect the output through the transformer in the same manner.

The relationship between the input voltage and the output voltage corresponding to equation (7) is given by the following equation.

(Equation 35) here

[Equation 36] If so, the phase of the input voltage and the phase of the output voltage are given as in equation (8).

Further, the description so far has been made on the assumption that the driving is performed by a sine wave, but a rectangular wave may be used. A square wave simplifies the transmission circuit, but a harmonic wave is superimposed on a signal other than the fundamental wave, so that a harmonic wave elimination circuit is required. However, there is also a well-known harmonic elimination method for simultaneously controlling a fundamental component of a voltage except for undesired harmonics in a rectangular wave. This is because one cycle of the above-mentioned rectangular wave is composed of a plurality of rectangular waves having different time widths, so that three cycles with respect to the fundamental wave are obtained.
It is a method to remove low order harmonics such as double and five times, and this method is described in detail in "Power Electronics & AC Drive" published by Denki Shoin by BKBose, translated by Hatsenguji, Naito, pages 174 to 176. The description is omitted here.

Although the case where the capacitance changes with displacement has been described above, the present invention is not limited to this case.
The case where the inductance is changed along with the displacement to make the capacitance constant may be employed. In this case, the change in the inductance is formed according to the position between the magnetic material entering and exiting the coil. A commercially available differential transformer type displacement meter is based on this principle, but details are omitted.

In the above description, the case where the capacitance is proportional to the displacement has been described. However, the present invention is not limited to this case. Alternatively, a type in which the capacitance between the electrode 1 and the electrode 2 of the displacement sensor is measured using a metal plate to be measured as an intermediate electrode may be used. In this case, the capacitance is

(37) It changes in inverse proportion, like

In this case,

(38) Then, by substituting into equation (12) and approximating as
And the following equation is obtained as an equation corresponding to equation (16).

[Equation 39] Similarly, since the phase difference is proportional to the minute displacement, the above discussion can be applied as it is.

[0035]

As described above, according to the present invention, an electric resonance circuit is formed by using a capacitance or an inductance which changes with displacement, and a frequency applied to the resonance circuit is set to a predetermined value to set a resonance point. In addition to calculating the displacement from the proportional relationship between the phase and the displacement in the vicinity, when the phase exceeds a predetermined range, the applied frequency is adjusted by a predetermined value so that the phase change falls within a predetermined range, and the frequency corresponds to the displacement. Is calculated, the phase shift amount at the latest applied frequency is counted, and the displacement is obtained from the counted value, so that the measurement range can be enlarged.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a capacitance between two metal plates facing each other.

FIG. 2 is an explanatory diagram of capacitance of two metal cylinders arranged coaxially.

FIG. 3 is an explanatory diagram of capacitance when a metal plate to be measured is used as an intermediate electrode.

FIG. 4 is a diagram illustrating the principle of detection of a displacement sensor according to the present invention.

FIG. 5 is a phase characteristic diagram when an applied frequency changes.

FIG. 6 is a phase-displacement characteristic diagram of the displacement sensor according to the present invention.

FIG. 7 is a phase-displacement characteristic diagram of the same displacement sensor when it is far away from a reference position.

FIG. 8 is a phase-displacement characteristic diagram of the displacement sensor when the set position interval is set large.

FIG. 9 is a phase-displacement characteristic diagram of the displacement sensor when a reference frequency is determined so that an overlap region exists.

FIG. 10 is an explanatory diagram of a case where an inductance is configured by a transformer.

[Explanation of symbols]

 x displacement C capacitance L inductance R resistance

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F063 AA02 CA10 DA05 HA01 HA05 HA11 HA12 KA02 LA04 LA05 LA30 2F077 AA27 CC02 HH02 HH03 HH11 HH13 TT02 TT04 TT07 TT21 TT23

Claims (11)

[Claims] 1. A displacement sensor including a resonance circuit including a capacitor and a coil, at least one of which is adapted to change in response to a displacement, wherein an alternating current is applied to the resonance circuit to respond to a shift in resonance frequency. When the resulting phase change from the resonance circuit exceeds a predetermined range, the applied frequency is adjusted by a predetermined value to keep the phase change within the predetermined range, and the amount of addition and subtraction of the frequency corresponding to the displacement and the latest applied frequency Measuring the phase shift amount in step (a), and obtaining the displacement from the measured values. 2. The displacement sensor measuring method according to claim 1, wherein the capacitance of the capacitor is formed directly or via a relay electrode, depending on the facing area between the two electrodes, and according to the displacement. Wherein the facing area is changed. 3. The method for measuring a displacement sensor according to claim 1, wherein the inductance of the coil is formed in accordance with a position between the coil and a magnetic material entering and exiting the coil, and the position changes in accordance with the displacement. A displacement sensor characterized in that: 4. The displacement sensor according to claim 1, wherein the phase of the detection signal of the resonance circuit is determined based on the phase of the applied frequency. 5. The displacement sensor according to claim 4, wherein said phase difference is detected by digital phase detection means by discretely approximating said phase change range. 6. The displacement sensor according to claim 1, wherein the adjustment value of the applied frequency is an integral multiple of a predetermined value. 7. The displacement sensor according to claim 1, wherein the applied AC waveform is a rectangular wave having a constant amplitude. 8. The displacement sensor according to claim 7, wherein one cycle of the rectangular wave is constituted by a plurality of rectangular waves having different time widths, and has a predetermined lower order such as three times or five times the fundamental wave. Displacement sensor characterized by eliminating higher harmonics. 9. The displacement sensor according to claim 6, wherein the predetermined value corresponds to the phase change range according to claim 5. 10. The displacement sensor measuring method according to claim 1, wherein the applied frequency and the detected value of the discrete phase in claim 4 correspond to the displacement by using the integer value in claim 6 as a parameter. A displacement sensor characterized by being linearized as: 11. An applied frequency according to claim 1, wherein
A displacement sensor characterized in that the applied frequency is determined so that one or more approximate straight lines exist corresponding to a certain range of displacement.