JP2991798B2 - Capacitance type measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a length measuring device for converting a mechanical displacement into an electric signal as a change in capacitance.
The present invention relates to a length measuring device in which the relationship between the displacement and the capacitance is linear, and is not affected by the fluctuation of the dielectric constant of the dielectric between the electrodes.

[0002]

2. Description of the Related Art A conventional capacitance type length measuring device of this type has a structure as shown in FIG. FIG.
As shown in FIG. 4, the essential parts of this length measuring device are a cylindrical reference electrode 1 and a measuring electrode 2, a column-shaped common core electrode 3 concentrically arranged in the electrodes 1, 2, and an electrode. 1 and a cylindrical screen 4 movable along the center between the core electrode 3.

A reference square wave voltage Vr and a measured square wave voltage Vm are applied to the electrodes 1 and 2, respectively. The voltages Vr and Vm have the same frequency and a phase difference of 180 ° (opposite phase)
The voltage Vr is constant, and the voltage Vm is variable.

In the detector of the length measuring device having such a configuration, a measuring capacitor Cm (capacitance cm) is formed between the electrode 1 and the core electrode 3, and a measuring capacitor Cm is formed between the electrode 2 and the core electrode 3. A reference capacitor Cr (capacitance cr) is formed.

In this length measuring device, when the screen 4 is displaced and the capacitance cm of the measuring capacitor Cm changes, the measuring square wave voltage Vm changes by the electronic device so that the AC voltage induced on the core electrode 3 becomes zero. Is done. That is, this results in a reference square wave voltage V
r, a current generated in the core electrode 3 by applying r,
Applying the measurement square wave voltage Vm to the reference capacitor Cr changes the measurement square wave voltage Vm so that the sum with the current generated in the core electrode 3 becomes zero. From this relation, a relational expression such as Equation 1 is established.

[0006]

(Equation 1)

Here, in equation (1), the voltages Vm and Vr have opposite signs because their phase difference is 180 °, and Vr = −
When expressed as Vr ′, Equation 2 is obtained.

[0008]

(Equation 2)

In equation (2), the proportionality constant Vr '/ cr is positive, and the relationship between the capacitance cm of the measuring capacitor Cm and the measuring square wave voltage Vm is such that as the capacitance cm increases, the measuring square wave voltage Vm also increases, and the capacitance cm Decreases, the measured square wave voltage Vm also decreases.

In FIG. 14, the screen 4 is moved so as to be inserted into the electrode 1 (moved rightward in the figure, X-axis direction).
Assuming that the displacement X at this time is positive, the measuring capacitor Cm
Is represented by Equation 3.

[0011]

(Equation 3)

Here, c ₀ represents the capacitance of the measurement capacitor Cm when the screen 4 is at the reference position (X = 0), and a is a proportional constant. Equation 2 above
From Equation 3 and Equation 3, the measured square wave voltage Vm can be expressed as a linear equation of displacement X as shown in Equation 4.

[0013]

(Equation 4)

As shown in Equation 4, the proportional constant -Vr'ac ₀
/ Cr is always a negative value, whereby the measured square wave voltage Vm decreases as the displacement X increases, as shown in FIG.
As the displacement X decreases, the measured square wave voltage Vm increases.

As described above, the displacement X of the screen 4 is
If the direction of insertion is positive, the relationship between the displacement X and the increase / decrease of the measured square wave voltage Vm is reversed, and the displacement X and the measured square wave voltage Vm are in a linear relationship but not in a proportional relationship. become.

In general, a spindle is directly connected to the screen 4, and the direction in which the spindle is pushed, that is, the direction in which the screen 4 is inserted into the electrode 1, is displayed as a positive value.

Therefore, in the relationship between the displacement X and the measured square wave voltage Vm as described above, when the measured square wave voltage Vm is measured by a voltmeter and the displacement X is displayed, the measured square wave voltage Vm and the displacement It is necessary to further convert the measured square wave voltage Vm so that its proportionality constant becomes positive while maintaining a linear relationship with X. For this reason, the conversion requires an electronic circuit such as an arithmetic circuit with high accuracy.

[0018]

As described above, in the conventional capacitance type length measuring device, it is necessary to use an electronic circuit in order to display the displacement X using the actually measured square wave voltage Vm. It is. The addition of such an electronic circuit not only leads to an increase in cost, but also causes a problem that the stability and temperature characteristics of the circuit are reduced.

Further, the adjustment of the measurement reference point in the conventional capacitance type length measuring device can be performed only by the electronic device.
When a plurality of length measuring devices are configured using the same electronic device, it is necessary to adjust the measurement reference point for each detecting unit of each measuring device.

Further, in the detection section of the conventional capacitance type length measuring device, since the reference electrode 1, the measurement electrode 2, and the screen 4 are each cylindrical, a boring process is used for the production thereof. In this boring process, a hole is formed in a workpiece, a boring tool is inserted into the hole, and the hole is cut and finished. Therefore, the diameter of the screen 4 and the core electrode 3 is reduced depending on the size of the boring tool. There was a limit. Also, the screen 4 is inserted into the core electrode 3,
Since the configuration is such that the core electrode 3 and the screen 4 are inserted into the reference electrode 1, there is a limit in reducing the diameter of the reference electrode 1. For this reason, there is room for inserting the detection unit in the space around the object to be measured, for example, in the vertical dimension (the Y-axis direction as the short direction when the longitudinal direction of the core electrode 3 is the X-axis direction). However, when there is no room for inserting the detection unit in the front and rear thickness dimensions (the Z axis direction when the longitudinal direction of the core electrode 3 is the X axis direction and the short direction is the Y axis direction in FIG. 14). However, there is a problem that the measurement object cannot be measured.

The present invention has been made to solve the above-mentioned problems, and eliminates the need for an electronic circuit for converting the proportionality constant, thereby reducing the cost by directly displaying the displacement using a square wave voltage measured. It is another object of the present invention to provide a capacitance-type length measuring device that can improve the stability and temperature characteristics of a circuit and reduce the thickness of a detection unit.

[0022]

According to the present invention, there is provided a capacitance-type length measuring device in which a core electrode, a measurement electrode, a correction electrode, a reference electrode, and a screen are formed in a plate shape, and the core electrode is interposed via a measurement capacitor. Is provided with a measurement electrode on one side, and a correction electrode and a reference electrode provided on the other side of the core electrode via a correction capacitor and a reference capacitor.

[0023]

According to the capacitance type length measuring device of the present invention, a measuring electrode is provided on one side of a core electrode, a correction electrode and a reference electrode are provided on the other side, and a flat screen enters a measuring capacitor by a spindle. When the capacitance changes, the measured square wave voltage is changed so that the feedback voltage induced to the flat core electrode by the electronic device becomes zero, and the measured square wave voltage is measured by a voltmeter or the like to measure the mechanical voltage. The actual displacement can be detected.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. 1 is a cross-sectional view showing a schematic configuration of an embodiment of the present invention, FIG. 2 is a front view, and FIG. 3 is a cross-sectional view taken along line BB of FIG.
In the figure, 11 is a flat measurement electrode, 12 is a flat correction electrode, 13 is a flat reference electrode, 14 is a flat core electrode, which is measured in parallel with one surface of the fixed core electrode 14. An electrode 11 is provided, and a correction electrode 12 and a reference electrode 13 are provided to face the other surface of the core electrode 14 in parallel. Then, a measurement capacitor is formed between the measurement electrode 11 and the core electrode 14, and the correction electrode 12
And a correction capacitor is formed between the core electrode 14 and
Further, a reference capacitor is formed between the reference electrode 13 and the core electrode 14. Reference numeral 15 denotes a screen fixed to a spindle 25 supported by bearings 20 and arranged in parallel between the measurement electrode 11 and the core electrode 14 by movement of the spindle 25. The screen 15 is electrically grounded,
17 and 18 which move in parallel along the core electrode 14 are screws for adjusting the capacitance provided on the correction electrode 12 and the reference electrode 13, 5 is a guide rod attached to the screen 15, and 6 is sliding of the guide rod 5. The guide members 7 and 8 are guide members having guide grooves 16 for guiding the electrodes.

[0025] These measurements capacitors, the capacitance of the correction capacitor and the reference capacitor c ₁ respectively,
c ₂ and c ₃ . When the spindle 25 is pushed in,
Screen 15 is configured to enter into the measurement capacitor. When the screen 15 is changed capacitance c ₁ of the measuring capacitor and moves with the spindle 25, the measurement square wave voltage E ₃ is varied as a feedback voltage Em induced in the core electrode 14 becomes zero by later-described electronic device . The direction in which the screen 15 enters the measuring capacitor is positive.

[0026] the measurement capacitor, the correction capacitor and the reference capacitor are consist of the same dielectric (air), the measurement electrode 11, the correction electrodes 12 and respective reference square wave voltage E ₁ to the reference electrode 13, the auxiliary square wave voltage E ₂ and measured square wave voltage E ₃ is applied. The reference square wave voltage E ₁ and the complementary square wave voltage E ₂ have the same frequency and a phase difference of 1
The square wave voltage is set to 80 ° (opposite phase), and the reference square wave voltage E ₁ and the measured square wave voltage E ₃ are set to the same frequency and the same phase square wave. The reference square wave voltage E
₁ and the complementary square wave voltage E ₂ are constant invariable voltages,
Measurements square wave voltage E ₃ is the voltage varied by the electronic device to be described later.

If the capacitance c ₁ when the displacement X of the screen 15 is zero is c ₀ , c ₀ | E ₁ | =
One or both of the complementary square wave voltage E ₂ and the capacitance c ₂ can be adjusted in advance so that the relationship of c ₂ | E ₂ | is satisfied.

The change of the displacement X and the measurement square wave voltage E ₃ of the next capacitive distance measuring device is a linear, yet be described that are in proportional relationship. As the spindle 25 moves and thereby displaces the screen 15 into the measuring capacitor, this capacitance c ₁ will also change. At the same time, the measured square wave voltage E ₃ is changed so that the feedback voltage Em induced in the core electrode 14 by the operation of the electronic device becomes zero. That is, assuming that the currents induced in the core electrode 14 by the measurement capacitor, the correction capacitor, and the reference capacitor are i ₁ , i ₂ , and i ₃ , the measurement square wave voltage E ₃ is changed so as to satisfy Expression 5.

[0029]

(Equation 5)

Equation 5 can be expressed as Equation 6.

[0031]

(Equation 6)

[0032] Thus, measuring the square wave voltage E ₃ can be expressed as in equation 7.

[0033]

(Equation 7)

The measured square wave voltage E ₃ and the reference square wave voltage E ₁
Is the in-phase square wave voltage and the measured square wave voltage E ₃
And the complementary square wave voltage E ₂ are square-wave voltages having opposite phases, and therefore, by replacing E ₂ = −E ₂ ′ and rewriting equation 7,
The measured square wave voltage E ₃ is as shown in Expression 8.

[0035]

(Equation 8)

On the other hand, the spindle 25 is moved rightward in FIG. 1, to represent the displacement X of the direction in which the screen 15 is inserted into the measuring capacitor as a positive, c ₀ the capacitance of the measurement capacitor when X = 0 Then, the capacitance c ₁ can be expressed by Expression 9.

[0037]

(Equation 9)

[0038] Here, b is distance between the measuring electrode 11 and the core electrodes 14 for forming a measuring capacitor, the measuring electrode 11, between the geometrical dimensions and capacitance c ₀ such width of the core electrode 14 and screen 15 electrodes It is a positive constant determined by the quotient c ₀ / ε divided by the dielectric constant ε of the dielectric.

By substituting equation 9 into equation 8, the measured square wave voltage E ₃ can be expressed by equation 10.

[0040]

(Equation 10)

Then, c ₀ E ₁ b / c ₃ of the _{equation (} 10 ) is expressed as α
Then, when (c ₂ E ₂ ′ −c ₀ E ₁ ) / c ₃ is replaced with β, Equation 10 is transformed into Equation 11.

[0042]

[Equation 11]

In this embodiment, c ₂ | E ₂ |
= C ₀ | E ₁ |, which is c ₂
Since it can be replaced with E ₂ ′ = c ₀ E ₁ , if this is substituted into Expression 11, β = 0. Therefore, the measurement square wave voltage E ₃ can be represented by the number 12.

[0044]

(Equation 12)

Since this proportionality constant α is a positive value, the relationship between the measured square wave voltage E ₃ and the displacement X is such that as the displacement X increases, the measured square wave voltage E ₃ is directly proportional to the displacement X as shown in FIG. When the displacement X decreases and the displacement X decreases, the measured square wave voltage E
₃ decreases in direct proportion to the displacement X, and when the displacement X is zero, the measured square wave voltage E ₃ also becomes zero. As shown in Equation 10, the measured square wave voltage E ₃ is c ₀ / c ₃ , c ₂ / c ₃
Therefore, if the measurement capacitor, the correction capacitor, and the reference capacitor are formed of the same dielectric, there is no influence from the dielectric constant. Also, c ₂ | E ₂ | ≠ c ₀ | E
_In the case of ₁ |, since β is a non-zero constant, the measured square wave voltage E ₃ increases by β, and the display value of the measured length is apparently such that the point of X = 0 is minus β / α. Will move to the side.

[0046] _{Thus, c 2 | E 2 | =} c 0 | E 1 |
, It is necessary to adjust one or both of the capacitance c ₂ and the complementary square wave voltage E ₂ as described above. To adjust the capacitance c ₂ , a large diameter adjusting screw 17 attached to the correction electrode 12 is used.
It is performed by

[0047] Generally, the electrical components such as good potentiometer temperature characteristics are used in the electronic apparatus side to adjust the auxiliary square wave voltage E _2, so not that no change even if the temperature changes, very When good temperature characteristics are required, it is preferable that there is no electric component such as a potentiometer. By providing a correction capacitor in the detection unit, it is possible to adjust the zero point of the displacement X without an electric component such as a potentiometer, and to adjust the zero point of the displacement X on the detection unit side without moving the entire detection unit. Possible and easy.

[0048] Incidentally, it is difficult to attach the electrical components such as potentiometer for changing the auxiliary square wave voltage E ₂ to the detector side. In addition, by providing the correction capacitor and the adjustment screw with the correction capacitor, the measurement origin of the detection unit can be made to coincide.

The capacitance c ₃ of the reference capacitor is adjusted by the adjustment screw 18 is performed. Any detector also by adjusting the capacitance c _3, alpha and can be the same value, can be connected without re-calibrating the different detector to a common electronic device.

FIG . 5 is a block diagram showing a circuit configuration of an electronic device for applying a voltage to the detecting section, and FIG. 6 is a time chart showing a phase relationship between the output voltages. In the figure, reference numeral 30 denotes an oscillator for outputting a reference square wave voltage Eosc. This oscillator 30 may be either a crystal type or a CR type. In the case of a crystal type, a high frequency is generally used. Try to get the desired frequency. By switching between the DC voltage Er and the ground level at the electronic switch 31 which is controlled by the output voltage Eosc of the oscillator 30 has acquired the reference square wave voltage E ₁ shown in FIG. 6, an oscillator 30
Newsletter auxiliary square wave voltage E ₂ shown in FIG. 6 DC voltage -Er in electronic switch 32 controlled by the output voltage Eosc with by switching between the ground level.

E ₄ shown in FIG . 6 is the feedback voltage Em or E
m ′ is an AC voltage amplified by the input amplifier 33, and this AC voltage E ₄ is generated every half cycle of the square wave voltage Eosc (t ₁₁ ,
demodulated by _{t 12, t 13, t 21} , t 22, t 23 ......) to the demodulator 34 is input to the differential integrator 35. If the demodulated signal is different from zero, the output DC voltage E ₀ of differential integrator 35 will change as a function of the amplitude and polarity of the demodulated voltage until the input to differential integrator 35 reaches zero.

The measured square wave voltage E ₃ is obtained by switching the electronic switch 36 between the output DC voltage E ₀ and a constant voltage (ground level in FIG. 5) with the output voltage Eosc of the oscillator 30. Therefore, like the output DC voltage E ₀ , the measured square wave voltage E ₃ also changes until the input to the differential integrator 35 reaches zero. The measured square wave voltage E ₃ thus obtained is directly proportional to the displacement of the screen 15 as described above.

[0053] Further, appeared transient state by undesirable joining and time delay in the output DC voltage E _0, the transient state is generated at the switching point of the square wave (side), attenuated after a certain time. This transient state reduces the stability of the output DC voltage E _{0 and} the stability of the measured square wave voltage E ₃ ,
Decreases the stability of the support of the length measuring device. For this reason, it is necessary to take measures to eliminate the influence of the transient state in a length measuring instrument that requires a very high degree of stability. This transient can get a good measure square wave voltage E ₃ of it can stability be removed by providing a transient suppressor between the input amplifier 33 and the demodulator 34.

FIG . 7 is a block diagram showing a circuit configuration of an electronic device for applying a voltage to the detection unit when an excessive suppressor is provided.
FIG. 8 is a time chart showing the phase relationship between the output voltages. Reference square wave voltage E ₁ is able to switch the electronic switch 31 which is controlled by the frequency divider 38 to output voltage EOSC, divided by 2 the time delay circuit 37 and a frequency between the DC voltage Er and the ground level Have gained. The resulting complement square wave voltage E ₂ is the DC voltage -Er the output voltage between the ground level EOSC, the electronic switch 32 which is controlled by the frequency divider 38 to the circumferential halving the time delay circuit 37 and a frequency ing.

In the simplest case, the transient suppressor 39 is an electronic switch, and the output voltage E which is the clock signal of the oscillator 30 is output.
part is not a transient state for each one period of osc (t ₀₁ ~t _{02,
t 11 ~t 12, t 21 ~t} 22, ......) passes only. The signal without this transient state is input to the demodulator 34, and is demodulated every period (t ₀ , t ₂ , t ₄ ,... And t ₁ , t ₃ , t _5, ...) Of the output voltage Eosc. It is input to the dynamic integrator 35. If the demodulated signal is different from zero, the output DC voltage E ₀ of differential integrator 35 will change as a function of the amplitude and polarity of the demodulated voltage until the input to differential integrator 35 reaches zero.

The measured square wave voltage E ₃ is an output voltage Eos between the output DC voltage E ₀ and a constant voltage (ground level in FIG. 7).
c is obtained by an electronic switch 36 controlled by a time delay circuit 37 and a frequency dividing circuit 38 for dividing the frequency by half. Therefore, like the output current voltage E ₀ , the measured square wave voltage E ₃ also changes until the input to the differential integrator 35 reaches zero. The measured square wave voltage E ₃ thus obtained is proportional to the displacement of the screen 15 and is not affected at all by the transient state of the AC voltage E ₄ .

[0057] feedback voltage Em is basically induced into the fixed core electrode 14 should not affected by the excitation square wave voltages _{E 1, E 2, E 3} , The excitation square wave voltage E ₁ , E ₂ , and E ₃ must also not affect each other. Therefore, the lines 21, 22, 23, and 24 connecting the detection unit and the electronic device are shielded. When a correction capacitor and a reference capacitor are arranged in the detection unit, the shield of the line 24 to which the feedback voltage Em is led can be simplified by connecting the input side of the impedance transformer 26 and the discharge resistor 27 to the core electrode 14 as shown in FIG. Are connected, the other side of the discharging resistor 27 is grounded, and the output side of the impedance transformer 26 is connected to the input amplifier 33 of the electronic device.
Thereby, the impedance deformer 26 and the input amplifier 33
Can be reduced, the shield can be simplified, and when high sensitivity and high accuracy are not required, the square lines 21 to which the excitation square wave voltages E ₁ , E ₂ , E ₃ , and Em ′ are led are connected. , 22, 23, and 24 can be collectively shielded. However, when high precision and high sensitivity are required, the lines 21 and 21 where E ₁ , E ₂ , E ₃ , and Em ′ are led are provided.
The combined use of one shield of each of 2, 23, 24 further stabilizes the shield.

As described above, in the above embodiment, the plate-shaped core electrode 14 is interposed therebetween, the plate-shaped measurement electrode 11 on one side, the plate-shaped correction electrode 12 and the plate-shaped reference electrode on the other side. 13, the thickness dimension before and after in the Z-axis direction when the longitudinal direction of the spindle 25 in FIG. 1 is the X-axis direction and the short side direction is the Y-axis direction is compared with that of the conventional detection unit having a cylindrical electrode. In the past, even in places where the space around the measured object in the thickness direction was so small that the detector could not be inserted, there is a possibility that the detector can be inserted and the measured object can be measured. There is a growing effect.

In the above embodiment, the flat core electrode 1
4 in the middle, a plate-like measuring electrode 11 on one side,
The flat correction electrode 12 and the flat reference electrode 13 are provided on the other side. However, as shown in FIG. 10, the measurement electrode 11, the flat correction electrode 12 and the flat correction electrode 12 are provided on one side of the flat core electrode 14. The flat reference electrode 13 is opposed in parallel, and the measurement electrode 11 and the core electrode 14 serve as a measurement capacitor, the correction electrode 12 and the core electrode 14 serve as a correction capacitor, and the reference electrode 13 and the core electrode 14 serve as a reference capacitor. It may be formed. In this case, the thickness dimension before and after in the Z-axis direction and the width dimension before and after in the Y-axis direction could be made thinner than the conventional detection unit having a cylindrical electrode. Even in a place where the space in the thickness direction and the space in the width direction around the measured object (Y-axis direction in FIG. 10) are so small that the detecting unit cannot be inserted, the detecting unit can be inserted and the measured object can be measured. There is an effect that the possibility increases.

[0060] The measurement electrodes 11, correction electrodes 12, reference electrode 13, by the material of the core electrodes 14 for the same, it is possible to improve the temperature performance of the detector.

[0061]

As described above, according to the present invention, the measurement electrode is provided on one side of the core electrode, the correction electrode and the reference electrode are provided on the other side, and the flat screen enters the measurement capacitor by the spindle. When the capacitance changes, the measured square wave voltage is changed so that the feedback voltage induced to the flat core electrode by the electronic device becomes zero, and the changed measured square wave voltage is measured with a voltmeter or the like. Since it is configured to detect the amount of mechanical displacement, it eliminates the need for an electronic circuit to convert the proportionality constant and directly displays the displacement using a square wave voltage, thereby reducing costs and improving circuit stability and This has the effect of improving the temperature characteristics and reducing the thickness of the detection section.

[Brief description of the drawings]

FIG. 1A is a diagram showing one embodiment of a detection unit according to the present invention;
FIG. 3 is a sectional view taken along line A.

FIG. 2 is a front view showing an embodiment of the capacitance type length measuring device.

FIG. 3 is a sectional view taken along line BB of FIG. 2;

FIG. 4 is a characteristic diagram showing a relationship between a measured square wave voltage and displacement according to the present invention.

FIG. 5 is a block diagram showing an example of an electronic device of the capacitance type length measuring device of the present invention.

FIG. 6 is a time chart showing the phase relationship of FIG. 5;

FIG. 7 is a block diagram showing another example of the electronic device of the capacitance type length measuring device of the present invention.

FIG. 8 is a time chart showing the phase relationship of FIG. 7;

FIG. 9 is a circuit diagram in which an impedance transformer and a discharge resistor are attached to a detection unit.

FIG. 10 shows another embodiment of the detection unit of the present invention.
FIG. 4 is a sectional view taken along line FF of FIG.

FIG. 11 is a front view showing another embodiment of the capacitance type length measuring device.

FIG. 12 is a sectional view taken along line GG of FIG. 11;

FIG. 13 is a sectional view taken along line HH of FIG. 11;

FIG. 14 is a cross sectional view showing an example of a conventional capacitance type length measuring device.

FIG. 15 is a characteristic diagram illustrating a relationship between a measured square wave voltage and a displacement of the detection unit in FIG. 14;

[Explanation of symbols]

 11 Measurement electrode 12 Correction electrode 13 Reference electrode 14 Core electrode 15 Screen 25 Spindle

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01B 7/00-7/34

Claims (1)

(57) [Claims] 1. A correction capacitor is formed between a core electrode from which a feedback voltage is induced and a measurement electrode to which a reference square-wave voltage is applied, and a correction capacitor is formed between the core electrode and the measurement electrode. A correction electrode to which a complementary square wave voltage of opposite phase is applied at the same frequency as the reference square wave voltage, and a reference capacitor formed between the correction electrode and the core electrode, and which is in phase with the same frequency as the reference square wave voltage A reference electrode to which the measurement square wave voltage is applied, and a spindle fixed to transmit mechanical displacement, which is interposed between the measurement electrode and the core electrode and is parallel along the measurement electrode and the core electrode. A moving screen, and varying the measurement square wave voltage so that when the screen enters the measurement capacitor and the capacitance changes, the feedback voltage induced at the core electrode becomes zero. In the capacitance type length measuring device provided with a slave device, the core electrode, the measurement electrode, the correction electrode, the reference electrode, and the screen are formed in a plate shape, and the core electrode, the measurement electrode, the reference electrode, and the screen are formed on one side of the core electrode via the measurement capacitor. A capacitance-type length measuring device comprising: the measurement electrode; and the correction electrode and the reference electrode provided on the other side of the core electrode via the correction capacitor and the reference capacitor.