JPH03180716A - Electrostatic capacity type length measuring instrument - Google Patents

Abstract

PURPOSE:To connect the electrostatic capacity type length measuring instrument to an electronic device without being affected by changes in measurement environment nor performing calibration by varying a measured rectangular voltage so that a feedback voltage induced at a common core electrode is zero. CONSTITUTION:When a spindle 15 moves and a screen 13 is displaced to the right, capacitors C1 and C2 vary in capacitance values c1 and c2. The electronic device operates and the measured rectangular voltage E3 is varied so that the feedback voltage Em induced at the common core electrode 12 becomes zero. This voltage E3 is proportional to displacement X and not affected by variation in dielectric constant on condition that the capacitors C1 - C3 are made of the same dielectric. Namely, the capacitance values c1 and c2 vary only when the screen 13 move along the center axis of ring electrodes 9 - 11 and the electrode 12. Therefore, even if the spindle 15 has radial play, the movement of the screen 13 in this direction exerts a little influence upon the capacitance values c1 and c2.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a capacitive length measuring device that converts mechanical displacement into an electrical signal as a capacitance change.
In particular, it relates to a length measuring device that is not affected by variations in the dielectric constant ρ between electrodes.

<Prior Art> As a conventional capacitance type length measuring device of this kind, there is one having a detection section as shown in FIG.

The detecting section of this length measuring device consists of a ring electrode 1.2 which is arranged adjacently through an insulating sheet 5, and a spindle 7 which is arranged concentrically within this ring electrode 1.2 and supported by a bearing 8. A cylindrical common core electrode 3 attached to the outer periphery via an insulating material 6, and this common core electrode 8i! 3, and a lead wire 4 for taking out the current induced in the lead wire 3.

Ring electrode 1.2 and common core electrode 3 in this length measuring device
Capacitors C, C7 are formed between them, respectively.
The dielectrics of these capacitors C, , C, are made of the same material (air).

Further, constant sinusoidal voltages V1 and V having the same frequency, a phase difference of 180 degrees (opposite phase), and equal peak values are applied to the ring electrodes 1.2, respectively.

In this length measuring device, when the spindle 7 moves and the common core electrode 3 moves, a current i is generated in the common core electrode 3, and if the capacitance of the capacitor CllCr is C,,C, then the following A relationship is established.

C@Vs + Cr Vr = a 1
m where a is a positive proportionality constant and V, = -V.

Therefore, the current i generated in the common core electrode 3.

is 1 a = Vr (Cr C's) / a'
...It can be expressed as (1).

In FIG. 6, X indicates the displacement of the spindle 7, that is, the common core electrode 3. Here, the displacement X in the direction in which the spindle 7 is inserted into the ring electrode 1.2 (rightward in the figure) is positive, the displacement X in the direction in which it is extracted (leftward in the figure) is negative, and C3 If the displacement

c, = -εb, X+co ... (2) c, = εb
rX+Cr)... (3) This ε is the permittivity of the dielectric material of the capacitors C, , C, and b1 and br are positive proportionality constants.

Next, by substituting these equations (2) and (3) into equation (1), we get i', = V, ε(b, +b,)X... (4), and even if the dielectric constant ε is constant, In this case, the current in is directly proportional to the displacement X. Therefore, by converting this current i with an electronic device, the amount of displacement X can be obtained, and in the conventional length measuring instrument, the amount of displacement X was obtained and displayed as described above.

<Problems to be Solved by the Invention> In the conventional example described above, the length is measured by detecting the current i1 flowing through the common core electrode 3, which changes in proportion to the displacement X of the spindle 7. This requires that the dielectric constant ε of the dielectric in the capacitors C, , C, be constant.

That is, the dielectric material of the capacitors C, C, in the conventional example is air, and when the measurement environment, for example, temperature, humidity, density (atmospheric pressure) changes, the dielectric constant changes, and the current 1ffi for the same displacement X changes. The value of will change.

For this reason, when making precise measurements, it has been necessary to calibrate it in accordance with the measurement environment.

In addition, the proportionality constant S1 in equations (2>, (3), and (4))
b, are capacitors C,,C, respectively.

is a constant determined by the geometrical dimensions of the electrodes, that is, the inner diameter dimension of the ring electrode 1.2, the outer diameter dimension of the common core electrode 3, and the spacing thereof.

Therefore, unless the inner diameter of this ring electrode flil, 2 and the outer diameter of the common core electrode 3 are made strictly the same, the same displacement
It is not possible to obtain the same current i for both.

However, this is very difficult, and it has therefore been necessary to calibrate the electronic device for each detection section.

Furthermore, if a detection unit is attached to each channel of a two-channel electronic device and a sum-difference calculation is performed, it is necessary to calibrate each channel according to each detection unit as described above. There was also the problem that accurate measurements would not be possible if the detection unit was sometimes connected to a channel that was not tuned to the one that was tuned.

The present invention provides a capacitive length measuring device that is not affected by changes in the measurement environment such as temperature, humidity, and atmospheric pressure, and that has a detection section that can be connected to an electronic device without calibration. The purpose is to

Means for Solving the Problems> The capacitive length measuring device of the present invention comprises first and second cylindrical ring electrodes arranged on the same center line and adjacent to each other in an insulated state; A cylindrical third ring electrode arranged on the same center line as the third ring electrode, a cylindrical common core electrode arranged concentrically with these, and a center axis between the first and second ring electrodes and the common core electrode. and a screen having a window movable along the screen, and the first and second ring electrodes and the common core electrode are respectively provided between the first and second ring electrodes and the common core electrode through the window of the screen. A second measurement capacitor C,,C2 is formed, and a reference capacitor C is formed between the third ring electrode and the common core electrode.
3 is formed.

A first reference square wave voltage E+ and a second reference square wave voltage E2 having the same frequency and opposite phase are applied to the first and second ring electrodes, respectively.

Further, a measurement square wave voltage E3 having the same frequency and phase as the first or second reference square wave voltage is applied to the third ring electrode.

Further, the first and second reference square wave voltages E1 and E2 are the capacitance c10 of the first measurement capacitor C1 when the displacement X is zero and the first reference square wave voltage E1 when the displacement X of the screen is zero. The product c10E I and the capacitance C2 when the displacement X of the second measurement capacitor C2 is zero.
0 and fJjczoEz of the second reference square wave voltage E2 is set to be zero.

Furthermore, the measured square wave voltage E3 is such that when the capacitances Cl, C2 of the first and second measuring capacitors C, C2 change due to the displacement of the screen, the feedback voltage E1, induced in the common core electrode becomes zero. , by switching alternately between a constant DC voltage and a variable measured DC voltage Eo by means of an electronic device.

This electronic device includes a transient suppressor for removing transients in the feedback voltage E, a demodulator for demodulating its output signal, and a demodulator for demodulating the amplitude of the demodulated signal until it becomes zero when the demodulated signal is not zero. A measured DC voltage E that varies as a function of polarity. It is equipped with a differential integrator that outputs .

<Function> In the capacitive length measuring device of the present invention, the measurement square wave voltage E3 is varied so that the feedback voltage E1 induced in the common core electrode becomes zero.

That is, in order to make the feedback voltage E, zero, the first and second measuring capacitors C, , C2 and the reference capacitor C3 each generate a measuring square wave such that the sum of the currents induced in the common core electrode becomes zero. Vary the voltage E3.

In the present invention, the first and second measurement capacitors C,
Since the first and second reference square wave voltages E1 and E2 respectively applied to c2 are voltages with opposite phases, the currents induced thereby cancel each other out.

Further, when the displacement X is zero, the current induced by the capacitor CtC2, that is, CtoEl and C20E2 are set to be equal.

Under this condition, when the displacement X is zero, the sum of the currents induced by the first and second measuring capacitors C, C2 will be zero. Therefore, in order to make the current induced by the reference capacitor C3 zero, i.e. because C3E3=O,
The measured square wave voltage E3 becomes zero.

Also, the screen is displaced and its window moves, and the first and second measurement capacitors C1 and C2 formed through this window are
When one of the capacitances CI+C2 decreases and the other increases, the absolute value of the sum of the currents induced by them increases. Therefore, in order to reduce the total induced current to zero,
It is necessary to increase the measuring square wave voltage E3 applied to the reference capacitor C3 to induce a current in a direction that cancels the sum of the induced currents by the first and second measuring capacitors C, C2, and this voltage E, Increased by electronic devices.

In this manner, the length measuring device of the present invention has a proportional relationship in which the measured square wave voltage E gradually increases from zero in accordance with the displacement of the screen.

In particular, the measured square wave voltage E in the present invention is determined as a capacitance ratio as described later, so it is not affected by changes in the dielectric material, and the first to third ring electrodes and the common core electrode are not movable. The positional relationship between the two does not change, and by simply adjusting the capacitance c3 of the reference capacitor C3, it is possible to measure so that the same measured square wave voltage E3 can be obtained for the same displacement. With this adjustment, there is no need to calibrate errors even if multiple detection units are connected to the same electronic device.

Furthermore, in particular the electronic device according to the invention is provided with a transient suppressor to eliminate transients occurring in the feedback voltage Em at switching points of the square wave voltage.

<Example> The present invention will be described in detail below based on the drawings.

FIG. 1 is a schematic explanatory diagram showing the structure of a detection section of a capacitive length measuring device according to an embodiment of the present invention.

9.10.11 are cylindrical first to third ring electrodes. The first to third ring electrodes 9 to 11 are arranged so that their centers are located on the same center line.

Further, the first and second ring electrodes [, 9, 10 are arranged adjacent to each other with a ring-shaped insulator 14 interposed therebetween.

12 is a cylindrical common core electrode, and the first to 13th are screens, which are integrally attached to the spindle 15 supported by a bearing 16.

This screen 13 is cylindrical in shape, as shown in the cross-sectional view of FIG.

Further, this screen 13 is displaced along the center line between the first and second ring electrodes 9 and 10 and the common core electrode 12 as the spindle 15 is displaced.

The above first and second ring wires T69.10 and common core wires i
12, first and second measurement capacitors C, C2 are formed respectively through the window 13a of the screen 13, and a reference capacitor is formed between the third ring electrode [11 and the common core electrode 12]. C3 is formed. The capacitances of these first and second measurement capacitors C, C2 and reference capacitor C3 are c1, C2 and C3, respectively;
The capacitances C1 and C2 are varied by changing the area of the first and second ring electrodes i9, 10 and the common core electrode 12 which face each other through the window 13a due to the displacement of the screen 13.

Note that the dielectric materials in the capacitors C1 to C3 in this embodiment are all air.

Also, the first to third ring electric f! First and second reference square wave voltages E1 and E2 and measurement square wave voltage E3 are applied to voltages 9 to 11, respectively.

This first reference square wave voltage E1 is always constant, and the second reference square wave voltage E1 is always constant.
The reference square wave voltage E2 has the same frequency as the voltage E1, and is set to a voltage of opposite phase with a phase difference of 180 degrees, and this is also always kept constant.

The measured square wave voltage E is a voltage having the same frequency and the same phase as the voltage E1 or C2, and in this embodiment, it is in the same phase as the voltage E1. Note that this voltage E3 is connected to the capacitor C1.
The feedback voltage E induced in the common core electrode 12 by ~C3
, n are varied by an electronic device, which will be described later, so that they become zero.

Furthermore, if the capacitances C1 and C2 when the displacement X of the screen 13 is zero are respectively c10+020, then c
The voltage E1 or C2 is adjusted in advance so that 10E I + C20E 2 =○.

Next, the relationship between the displacement X in the detection section of the capacitive length measuring device of this embodiment and the measured square wave voltage E3 will be explained using a mathematical formula.

When the spindle 15 moves and thereby the screen 13 is displaced to the right in FIG. 1, the capacitances CI1 and C2 of the capacitors C, C2 change.

At the same time, an electronic device to be described later operates to vary the measurement square wave voltage E3 so that the feedback voltage Effl induced in the common core electrode 12 becomes zero. That is, capacitor C
The currents induced in the common core electrode 12 by il, i2 . When i3, the measured square wave voltage E is varied so as to satisfy the following equation.

i1+i2 +i3 =O...(5) Ikuyapacita C1
The capacitances of ~C3 are C1~C3, respectively;
Since the voltages applied to the first to third ring electrodes 9 to 11 are E1 to E3, respectively, equation (5) can be expressed as follows.

c1E1+c2 C2+c3 C3=0 Therefore, the measured square wave voltage E3 can be expressed as follows.

C3= (CI Et +C2C2)/C3(6) On the other hand, when the spindle 15 moves to the right in FIG. 1 and the direction in which the screen 13 moves along with this is expressed as positive, then X Capacitor C when = 0,
, C2 are c10 and C20, respectively, so the capacitances Cl and C2 can be expressed by the following equations.

Ci =c+o(1b+X)... (7)C2=
c2o(1+b2X)... (8) Here it is 1, b2
are the first and second rings, respectively. Substituting equations (7) and (8) into equation (6), the measured square wave voltage E3 is: C3-(C+oE+ +C20E2 (b+ cl
OE1b2 c2gE2 )X)/C3... (9)
It can be expressed as

Since it is set in advance so that Cl0EI +C20E2 =O when X=O, under this condition, equation (9) becomes the following equation.

C3=clOEl (b, 10b2)X/C3(1
0) From equation (10), voltage E3 is proportional to displacement X, and its proportionality constant α is (2=c10El (t)+ +b2)/C3
(11) becomes.

As shown in equation (10), voltage E3 is expressed as a capacitance ratio such as C+o/c3, so if capacitors C1 to C3 are made of the same dielectric material, they will be affected by changes in dielectric constant. Not at all.

Further, in this embodiment, when the capacitance c3 of the capacitor C3 is adjusted, the proportionality constant α changes.

That is, by simply adjusting this capacitance c3, it is possible to make the proportional constant α equal to the same value, and it is possible to connect different detection units to a common electronic device without calibration.

To adjust the capacitance C3, first make a rough adjustment using the adjusting screw 18 with a large diameter, and then make a fine adjustment using the adjusting screw 19 with a small diameter.

Further, the capacitors CI to C3 in this embodiment are each composed of cylindrical first to third ring electrodes 9 to 11 and a cylindrical common core electrode 12,
The third ring electrodes 9 to 11 and the common core electrode 12 are fixed so that their central axes coincide.

Therefore, even if the central axes of the ring electrode and the core electrode are slightly shifted due to an impact, etc., the fluctuations in s1 and b2, which constitute the proportionality constant in equation (11), will be extremely small.
There is almost no effect on the voltage E3.Furthermore, in this embodiment, the capacitance CI+C2 changes only when the screen 13 moves along the central axis of the ring electrodes 9 to 11 and the common core electrode 12, and the spindle Even if radial play occurs in screen 15, such movement of screen 13 in the radial direction will hardly affect capacitance C1+C2.

In this regard, in the conventional type, the spacing between the common Z-electrode and the phosphor electrode changes due to the play of the spindle in the radial direction, which has a great effect on the capacitance.

Also, basically in this detection section, the common core electrode 12
The feedback voltage E induced in the excitation square wave voltage E1~
It is necessary to avoid being influenced by ES, and it is also necessary to prevent these voltages E, ~E3 from influencing each other.

Therefore, in this embodiment, voltages El to E3. Shield the wires 21 to 24 that connect the detection unit and the electronic device to guide Em.
It is shielded by

In particular, it is required that the line leading the feedback voltage E be completely shielded, but it is also possible to simplify the shielding by the following method.

That is, as shown in FIG. 3, the input side of the impedance transformer 26 and one side of the discharge resistor 27 are connected to the common core electrode 12 in the detection section, and the other side of the discharge resistor 27 is grounded. The output side of the impedance transformer 26 is connected to an electronic device. Thereby, the impedance between the impedance transformer 26 provided in the detection section and the electronic device can be reduced, and the shield can be simplified.

Therefore, when high sensitivity and high accuracy are not required, voltage E1. E2. It becomes possible to shield all the lines leading to E1, E, and = instead of one by one.

However, if high sensitivity and precision are required, it is desirable to shield each wire one by one.

In addition, by making the first to third ring electrodes 9 to 11 and the common core electrode 12 of the same material, the thermal expansion of the parts becomes the same, preventing unbalance of the capacitor due to temperature changes, and improving the temperature performance of the detection part. It is possible to improve the

FIG. 4 is a diagram showing the circuit configuration of an electronic device that applies a voltage to the detection section, and FIG. 5 is a time chart showing the phase relationship of the output voltage.

30 is an oscillator that outputs a square wave voltage EO20 as a reference. This oscillator 30 may be either a crystal type or a CR type, and in the case of a crystal type, a frequency divider may be used to obtain a desired frequency.

31 is a time delay circuit that delays the square wave voltage EO20 for a certain period of time, and 32 is a frequency divider circuit that divides the output signal into 1/2.

33.34 is an electronic switch, and the electronic switch 33 has a DC voltage Er and 7. ground level in response to a signal from the frequency dividing circuit 32 and outputs it as the first reference square wave voltage E1, and the electronic switch 34 outputs the DC voltage -E.
It also switches between r and ground level in response to a signal from the frequency divider circuit 32 and outputs it as a second reference square wave voltage E2.

35 is an input amplifier that inputs the feedback voltage E.

36 is a transient suppressor for removing a transient state generated in the voltage signal E4 outputted by the input amplifier 35, and in this embodiment, it is a square wave voltage E from the oscillator 30. It is composed of an electronic switch that connects and disconnects the voltage signal E4 in response to so.

A synchronous demodulator 37 inputs the voltage signal E4 from which the transient state has been removed, demodulates the signal from the frequency divider 32 every cycle, and outputs the demodulated signal.

38 inputs the demodulated voltage signal to generate a DC voltage signal.

It is a differential integrator that outputs .

Three are DC voltage signals. This is an electronic switch that switches between the voltage E3 and the ground level in response to a signal from the frequency divider 32, and outputs the measured square wave voltage E3.

In the above electronic device, the voltages E1 and E2 are square wave voltages having the same frequency and opposite phases because DC voltages of different polarities are intermittently output from the electronic switches 33 and 34 at the same timing.

Further, the voltage E3 is synthesized by outputting the DC voltage Eo from the differential integrator 38 intermittently from the electronic switch 39 at the timing of the voltage signal from the frequency divider 32.

Furthermore, the DC voltage E. is synthesized as follows.

First, the feedback voltage Em is amplified by the input amplifier 35 and applied to the excess suppressor 36 as a voltage E4.

In this voltage signal E4, a transient state that occurs in the feedback voltage E1 occurs at the switching point (too, t+o, t2°·) of the square wave. This transient state attenuates and becomes stable after a certain period of time, and in this embodiment, in the period from too to t01, between t+o□ t11, and t21 in FIG.
)” t This occurred between 21 and 21.

The transient suppressor 36 that receives this voltage signal E4 converts the input voltage signal E4 into a voltage E. ,. The signal is cut and output every half cycle. Electronic switches 33, 34 in this embodiment.
39 is set to switch states in response to a signal obtained by delaying the voltage Eosc by a predetermined time. Therefore, the square wave voltage output from these electronic switches, E,'
, E2. The timing of generation of E3 and the timing of generation of voltage EQSC are shifted by the amount of delay as described above. Therefore, if settings are made so that the transient state of the voltage signal E4 falls within the range of this generation timing shift,
The transient suppressor 36 can be turned off only while a transient condition is occurring in the voltage signal E4, thereby eliminating the transient condition in the voltage signal E4.

The voltage signal E4 from which the transient state has been removed in this manner is demodulated every cycle by the demodulator 37.

Further, if the demodulated voltage is not zero, the DC voltage Eo output by the differential integrator 38 is varied as a function of the amplitude and polarity of the demodulated voltage until the demodulated voltage reaches zero.

As a result, voltage E3 is DC voltage E. together with differential integrator 3
8 until the input voltage reaches zero.

As a result, the voltage E3 satisfies the relationship of equation (10), is directly proportional to the displacement X, and has a positive proportionality constant.

<Effects of the Invention> According to this embodiment, the measured square wave voltage E3 proportional to the displacement of the spindle is expressed as a ratio of capacitances, so if the dielectric materials of the three capacitors C1 to C3 are made the same, the dielectric It is completely unaffected by the rate.

Therefore, it is not affected by changes in the measurement environment and can perform highly accurate measurements without calibration or correction due to changes in dielectric constant.

Furthermore, since the central axes of the three ring electrodes and the common core electrode in the present invention are aligned and fixed, the influence of their positional deviations on the measured square wave voltage E3 can be almost eliminated. , it is possible to provide a detection unit having almost constant characteristics.

Furthermore, since the characteristics of a plurality of detection sections can be made uniform simply by adjusting the reference capacitor C5, a plurality of detection sections can be connected to a common electronic device without recalibration.

Furthermore, the positions of the three ring electrodes and the common core electrode are fixed, and the screen integrated into the spindle moves between them in the direction of the central axis, so even if radial play occurs in the spindle, it will not be affected. , there is almost no effect on the positional relationship between the ring electrode and the common core electrode or the position in the central axis direction of the screen window, and there is no drop in accuracy due to such backlash.

Further, by providing an impedance transformer and a discharge resistor in the detection section, when high sensitivity and high accuracy are not required, the lines leading to the voltages E, to E1, E, and - can be shielded together.

Furthermore, ring electrodes 9 to 9 constituting the capacitors C1 to C3
By making the materials of the common core electrode 11 and the common core electrode 12 the same, it is possible to improve the temperature performance of the detector.

Further, by providing a transient suppressor in the electronic device, the voltages E1 and E can be stabilized from a signal without a transient state, thereby further improving the stability of the length measuring device.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of a detection section of a capacitance type length measuring device according to an embodiment of the present invention, and FIG. 3 is a diagram showing a state in which an impedance transformer and a discharge resistor are attached to the detection section shown in FIG. 1, and FIG. 4 is a capacitive type according to an embodiment of the present invention. Figure 5 is a diagram showing the configuration of the electronic device of the length measuring device. Figure 5 is a time shield showing the voltage phase relationship in Figure 4. Figure 6 is a diagram showing the structure of the detection section of a conventional capacitive length measuring device. It is. 9 ・ 10 ・ 11 ・ 12 ・ 13 ・ 14 ・ 15 ・ 21° 30 ・ 33° 35 ・ 37 ・ 38 ・ I 2 ・First ring electrode, ・Second ring electrode, ・Third ring electrode, ・Common core electrode , ・Screen, ・Insulator, ・Spindle, 2'2, 23.24... Shield, ・Oscillator, 34.36.39... Electronic switch, ・Input amplifier, ・Demodulator, ・Differential integrator , ・First measurement capacitor, ・Second measurement capacitor, 3 I E! 2 3 E. -Reference capacitor, C3, c3...Capacitance, -First reference square wave voltage, -Second reference square wave voltage, -Measurement square wave voltage, -Feedback voltage. Figure 2 (A) ←C3 ←" Eagle diagram C entrance) A-A Cross section diagram ♂" Figure 2

Claims (3)

[Claims] (1) Cylindrical first and second ring electrodes arranged on the same center line and adjacent to each other in an insulated state, and a cylindrical ring electrode arranged on the same center line as the first and second ring electrodes. A third ring electrode, a cylindrical common core electrode arranged concentrically within the first to third ring electrodes, and a cylindrical common core electrode having a cylindrical shape with a window provided on its outer periphery;
an electrically grounded screen movable along their central axes between the ring electrode and the common core electrode, the first and second ring electrodes and the common core electrode being connected to each other through a window in the screen; a reference capacitor C_3 is formed between the third ring electrode and the common core electrode, and a first reference square capacitor C_3 is formed between the third ring electrode and the common core electrode. Wave voltage E_
1 is applied to the second ring electrode, and a second reference square wave voltage E_1 having the same frequency and opposite phase as the first reference square wave voltage E_1 is applied to the second ring electrode.
_2 is applied to the third ring electrode, and the first or second
When a measurement square wave voltage E_3 having the same frequency and the same phase as the reference square wave voltages E_1 and E_2 is applied and the displacement X of the screen is zero, the capacitance c_1_0 of the first measurement capacitor C_1 when the displacement X is zero is The product c_1_0E_1 of the first reference square wave voltage E_1 and the product c_2_0 of the capacitance c_2_0 when the displacement X of the second measurement capacitor C_2 is zero and the second reference square wave voltage E_2
A transient suppressor that sets the first and second reference voltages E_1 and E_2 so that the sum with E_2 becomes zero and removes the transient state of the feedback voltage E_m, a demodulator that demodulates the output signal thereof, and a demodulator that demodulates the output signal thereof. A differential integrator outputs a measured DC voltage E_0 which varies as a function of the amplitude and polarity of the signal until this signal becomes zero when the signal is not zero. Capacitance C_ of the first and second measurement capacitors C_1, C_2
1. When C_2 changes, the measured square wave voltage E_3 is changed by alternating between a constant DC voltage and a variable measured DC voltage E_0 such that the feedback voltage E_m induced in the common core electrode is zero. A capacitive length measuring device characterized by being variable. (2) The input side of the impedance transformer and one side of the discharge resistor are connected to the common core electrode, the output side of the impedance transformer is connected to the electronic device, and the other side of the discharge resistor is grounded. The capacitance type length measuring device according to claim 1. (3) The capacitive length measuring device according to claim 1 or 2, wherein the first to third ring electrodes and the common core electrode are made of the same material.