JPH0460401A - Capacitance-type length measuring machine - Google Patents

Abstract

PURPOSE:To improve temperature characteristics and stability and reduce cost by directly using square-wave voltage without using an electronic circuit for conversion so as to enable display of displacement and also enable adjustment of a measurement base point in a detecting unit. CONSTITUTION:Respective ring electrodes 9, 10, 11 for measurement, correction and reference are placed in a detecting unit so that their centers are positioned on the same center line, while a common core electrode 12 is placed so that it can be moved along the center axis in th electrodes 9, 10, 11. In addition, respective capacitors C1, C2, C3 for measurement, correction and reference are formed between the electrodes 9, 10, 11 and the electrode 12, respectively. Reference square-wave voltage E1, correction square-wave voltage E2 and measurement square-wave voltage E3 are applied to the electrodes 9, 10, 11, respectively. In this case if a direction wherein the electrode 12 enters the electrode 9 is assumed to be positive, a displacement amount X of the electrode 12 is detected from the voltage E3 which is proportional to the displacement amount X and also whose proportional coefficient is positive, so that it is possible to display the displacement and to adjust a measurement base point in the detecting unit, thereby improving temperature characteristics and stability and reducing cost.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a length measuring device that converts mechanical displacement into an electrical signal as a change in capacitance. This relates to a length measuring instrument in which the relationship is linear and the constant of proportionality is positive.

<Prior Art> A conventional capacitive length measuring device of this type has a structure as shown in FIG.

As shown in the figure, the main part of this length measuring instrument is a cylindrical reference voltage tf! l, a measuring electrode 2, a cylindrical common core electrode 3 arranged concentrically within this electrode 1.2, and an electric [! 1 and a cylindrical screen 4 that is movable along the central axis between the core electrode 3 and the core electrode 3.

A reference square-wave voltage ■, and a measurement square-wave voltage V, are respectively applied to the electrodes 1.2. This voltage V, ,V
, are square wave voltages with the same frequency and a phase difference of 180 degrees (opposite phase), the voltage (2) is constant, and the voltage (2) is variable.

In the detection section of the length measuring device having such a configuration, the electric i
A measurement capacitor C, (capacitance Cs+) is formed between the electrode 2 and the core electrode 3, and a reference capacitor C, (capacitance cr) is formed between the electrode 2 and the core electrode 3.

In this length measuring instrument, an electronic device measures a square wave voltage V so that when the screen 4 is displaced and the capacitance C1 of the measuring capacitor C changes, the AC voltage induced in the core electrode 3 becomes zero.

is changed. That is, this is the current generated in the core electrode 3 by applying the reference square wave voltage Vr to the measurement capacitor C1, and the measurement square wave voltage ■,
The purpose is to change the measured square wave voltage (2) so that the sum of the current generated in the core electrode 3 by applying the voltage (2) becomes zero.

From this relationship, the following relational expression is established.

c, v, 10C, V@ = O Van = V r Cm / Cr
'...(1) Here, in equation (1), the voltage V,
, V have different signs because their phase difference is 180 degrees, and ■.

=−v,', Vat ==V,'C,/cr −
(2>.

In this equation (2), the proportionality constant v,'/cr is,
The relationship between the capacitance C3 of the measuring capacitor C and the measured square wave voltage V is given by the capacitance C,
When increases, the voltage V also increases, and the capacitance C,
When , decreases, voltage ■ also decreases.

In FIG. 6, if the displacement X when the screen 4 is moved so as to be inserted into the electrode 1 (moved to the right in the figure) is positive, then the capacitance C1 of the measurement capacitor C1 is expressed by the following equation.

C,=CG (1-aX) =-aCg X+cg -(3) where c
(, represents the capacitance of the measurement capacitor C1 when the screen 4 is in the reference position (X=O), and a is a positive proportionality constant.

From the above equations (2) and (3), the measured square wave voltage (1) can be expressed as a linear equation of the displacement X as follows.

Vm - (Vr'acg X/Cr) + (Vr'C
.

/cr) ...(4)(4)
As shown in the equation, the proportionality constant −V, ' a c 6/
cr is always a negative value, and as a result, as shown in FIG. 7, when the displacement X increases, the measured square wave voltage V, decreases, and when the displacement X decreases, the voltage V, increases.

In this way, if the direction in which the screen 4 is inserted into the electrode 1 is positive, the relationship between increase and decrease in the displacement X and the measured square wave voltage V is reversed, and the displacement X and the voltage V are linear. Although there is a relationship, it is not a proportional relationship.

Generally, a spindle is directly connected to the screen 4, and the direction in which this spindle is pushed, that is, the direction in which the screen 4 is pushed is the electric f! The direction of insertion into 1 is displayed as a positive value.

Therefore, when measuring the voltage V with a voltmeter or the like to display the displacement X, what is the relationship between the displacement It is necessary to further convert the voltage V so that its proportionality constant becomes positive while maintaining the voltage V.

Therefore, this conversion requires electronic circuits such as highly accurate arithmetic circuits.

<Problems to be Solved by the Invention> As mentioned above, in the conventional capacitive length measuring device, an electronic circuit must be used to actually display the displacement X using the measured square wave voltage V1. is necessary. The addition of such an electronic circuit not only leads to an increase in cost, but also causes problems in that it becomes a factor that deteriorates the stability and temperature characteristics of the circuit.

In addition, adjustment of the measurement reference point in conventional capacitive length measuring instruments can only be performed using an electronic device, and when multiple length measuring devices are configured using the same electronic device, the measurement reference point can be adjusted using the same electronic device. It was necessary to match the detection section of each length measuring device.

The present invention makes it possible to display displacement directly using a square wave voltage without using the conversion electronic circuit in the conventional example, and also makes it possible to adjust the measurement reference point within the detection unit,
The object of the present invention is to provide a capacitance type length measuring device that reduces costs and improves temperature characteristics and stability.

<Means for Solving the Problems> The capacitive length measuring device of the present invention includes a cylindrical measuring ring electrode, a correction ring electrode, and a reference ring electrode arranged on the same center line, and A substantially cylindrical common core electrode that is movable along the central axis is provided, and a measurement capacitor C1 and a correction capacitor C are provided between the measurement ring electrode, the correction ring electrode, the reference ring electrode, and the common core electrode, respectively.
2 and a reference capacitor C5.

A reference square wave voltage E0, a supplementary square wave voltage E2, and a measurement square wave voltage E3 are applied to the measurement ring electrode, correction ring electrode, and reference ring electrode, respectively. This complementary square wave voltage E2 is set to have the same frequency and opposite phase as the reference square wave voltage E1, and the measurement square wave voltage Em is also set to have the same frequency and opposite phase as the reference square wave voltage E1. ing.

In addition, in this length measuring device, the displacement
The capacitance co of the measuring capacitor CI when is zero
The product Co1E5l of the reference square wave voltage E1 and the product C2E21 of the capacitance c2 of the correction capacitor C2 and the complementary square wave voltage E2 are set to be equal.

Further, when the capacitance c1 of the measuring capacitor C1 changes due to the displacement of the common core electrode, the electronic device changes the constant DCt pressure and the variable measuring DC voltage Eo so that the feedback voltage Em induced in the common core electrode becomes zero. The measured square wave voltage is varied by alternating between the two.

Effect> In the capacitance type length measuring device of the present invention, the measurement square wave voltage E3 is changed so that the feedback voltage Em induced in the common core electrode becomes zero.

That is, in order to make the feedback voltage Em zero, the measurement capacitor C
The measured square wave voltage E3 is varied so that the sum of the currents induced in the common core electrode by I, the correction capacitor C2, and the reference capacitor C3, respectively, becomes zero.

In the present invention, since the reference square wave voltage E1 and the complementary square wave voltage E2 applied to the measurement capacitor cl and the correction capacitor C2 are voltages with opposite phases, the voltages induced thereby cancel each other out. .

Furthermore, when the displacement X is zero, the current value induced by the measurement capacitor C9 and the correction capacitor c2, that is, CQ
IEtl and C21E21 are set to be equal.

Under this condition, when the displacement X is zero, the currents induced by the measurement capacitor CI and the correction capacitor c2 are equal, so their sum becomes zero.

Therefore, in order to make the current induced by the reference capacitor C3 zero, ie, 03 Eg =O, the measured square wave voltage Em becomes zero.

Also, when the common core electrode is displaced into the measuring ring electrode and the capacitance Cl of the measuring capacitor C1 increases, the induced current increases because the reference square wave voltage E1 is constant, and the current induced in the correction capacitor C2 increases. Since the current induced by is constant, the sum of the absolute values of the currents increases.

Therefore, in order to make the sum of induced currents zero, the measurement square wave voltage E3 applied to the reference capacitor C3 must be increased to induce a current in a direction that cancels the sum of the induced currents by the measurement capacitor C8 and the correction capacitor C2. is required, and this voltage Em is increased by the electronics.

In this manner, the length measuring device according to the present invention has a proportional relationship in which the measured square wave voltage Em gradually increases from zero in accordance with the displacement of the common core electrode.

<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, and 11 are a measuring ring electrode, a correction ring electrode, and a reference ring electrode, each having a cylindrical shape. These ring electrodes 9, 10, 11 are arranged so that their centers are located on the same center line.

Reference numeral 12 denotes a substantially cylindrical common core electrode, which has a mounting hole 12a formed along the central axis from its negative end face side.

A spindle 15 supported by a bearing 16 is inserted through a cylindrical insulator 14 into the mounting hole 12a of the common core electric wire 12 and is integrally attached thereto. The center axis of the core electrode 12 is configured to move on the center axes of the measurement ring electrode 9, the correction ring electrode 10, and the reference ring electrode fai11.

In this embodiment, the spindle 1 of the common core electrode 12
The end opposite to the end where 5 is attached is the measuring ring electric 8i! When the spindle 15 is pushed in, the end of this common core electrode 12 enters into the measuring ring electrode 9, and this direction is set as positive. Further, even if the common core electrode 12 moves, it always penetrates the correction ring electrode 10 and the reference ring electrode 11, and its length and position are adjusted so that the area and distance facing these electrodes 10 and 11 do not change. has been decided.

A measurement capacitor C1, a correction capacitor C2, and a reference capacitor c1 are formed between the measurement ring electrode 9, the correction ring electrode 1O, the reference ring electrode 11, and the common core electrode 12, respectively. The capacitances of the measurement capacitor C1, correction capacitor C2, and reference capacitor C3 are C1, C3, and C3, respectively. It is variable by changing.

Further, a reference square wave voltage E1, a supplementary square wave voltage E2, and a measurement square wave voltage E3 are applied to the measurement ring electrode 9, the correction ring electrode IO, and the reference ring electrode 11, respectively. This reference square wave voltage Em is always constant, and the complementary square wave voltage E2 has the same frequency as this voltage E1, and is set to an opposite phase voltage with a phase difference of 180 degrees. always kept constant. The measurement square wave voltage E3, like the voltage E2, has the same frequency and opposite phase as the voltage Em, and is varied by an electronic device to be described later.

This measurement square wave voltage E3 is variable by three capacitors 0
1 to C9, the feedback voltage Em induced in the common core electrode 12 is varied by the electronic device so that it becomes zero.

Furthermore, in this embodiment, the displacement X of the common core electrode 12
If the capacitance of capacitor C1 when is zero is CQ, then complementary square wave voltage E2 and capacitance c2 are set in advance so that CQ I E+ I = C21E21.
Either one or both are being adjusted.

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.

The spindle 15 moves, thereby causing the common core electrode 12
When C1 is displaced to the right in FIG. The measurement square wave voltage E3 is varied.

That is, if the currents induced in the common core electrode 12 by the capacitors C to C3 are respectively 11.12 and i3,
The measured square wave voltage E3 is varied so as to satisfy the following equation.

il +i2 +i3 =O...(5) Capacitor 0
The capacitances of 1 to C3 are C1 to C3, respectively, and the voltages applied to the measurement ring electrode 9, the correction ring electrode 1Filo, and the reference ring electrode 1fll are Em to E, respectively.
Since m, equation (5) can be expressed as follows.

c) Em +c2 E2 +C3E3 ==0 Therefore,
The measured square wave voltage E3 can be expressed as follows.

E3-- (c+Em+C2E2)/C3...(
6) The voltages E3 and El are square wave voltages with opposite phases, and the voltage Em
and E2 are in-phase square wave voltages, so E1'--
If Et is replaced and equation (6) is rewritten, the measured square wave voltage E3 becomes as follows, E3-(c+El'
C2E2 )/C3-(7) On the other hand, spindle 15
moves to the right in FIG. 1, the direction in which the common core electrode 12 enters the measuring ring voltage [9] is set as positive, and the capacitance of the measuring capacitor C2 at X-0 is C8, then the capacitance C1 is It can be expressed by the following formula.

C1=co (1+bX)...(8'
) where b is the inner diameter of the measuring ring electrode 9 forming the measuring capacitor C1, the outer diameter of the common core electrode 12 and the capacitance c (, by the quotient CO/ε, divided by the dielectric constant ε of the dielectric of the capacitor C1). It is a fixed value and is a positive constant.

Substituting this equation (8) into equation (7), the measured square wave voltage E
3 is E3 = (bcoE1'X/cs) + (co E
tC2E2)/c) ...(9), α=bcoE1'/c3, β-(c,, E+'C2E2)/C3-(10
), equation (9) is transformed as follows.

E3-aX+β...(11) In this example, it is set so that C2l E2 l ==(ol E, which is C2E2=coEm'
By substituting this into equation (10), β=0. Therefore, the measured square wave voltage E3 can be expressed by the following equation.

E3-aX=c6 E1'bX/c3 ++, (12)
Since this proportionality constant α is a positive value, the relationship between voltage E3 and displacement X is as shown in FIG. When the voltage Em decreases, the voltage Em decreases in direct proportion to the displacement X, and when the displacement X is zero, the voltage E3 also becomes zero.

Furthermore, as shown in equation (9), the voltage E3 is C6/C3,C
Since the capacitance ratio is 2/C3, if the capacitors 01 to c1 are made of the same dielectric material, they will not be affected by the dielectric constant at all.

Also, in the case of C21E21≠Co IEI l, β is a non-zero constant, so the voltage E3 increases by β, and the displayed value of the measured length appears to be at the upper X-0 point by β/α. It will move to the negative side.

Therefore, it is necessary to set the relationship C21E21=co IEI 1, but in order to set it like this, it is necessary to adjust one or both of the capacitance C2 and the voltage E2 as described above. be. In this embodiment, in order to adjust the capacitance c2, the correction
Large diameter adjustment screw 4 attached to Gummy pole 10
Make approximate adjustments with 2, then use the adjustment screw 43 with the smaller diameter.
Make fine adjustments.

In addition, when adjusting the voltage E2, electrical components such as potentiometers with good temperature characteristics are generally used on the electronic device side, but if even stricter temperature characteristics are required, such electrical components are used. It is preferable not to use parts. In this case as well, in this embodiment, by adjusting the correction capacitor C2, the voltage E2 is
It is possible to adjust the X-0 point without adjusting. Also, this correction capacitor C2 and its adjustment screws 42 and 43
By providing this, even when a plurality of detection units are used simultaneously, it is possible to make the measurement origins of all of them coincide.

Furthermore, the capacitance C5 of the reference capacitor C3 in this embodiment is adjusted by making an approximate adjustment with the large adjustment screw 18 and then making a fine adjustment with one small adjustment screw.

Therefore, the capacitance c3 in the plurality of detection sections can be adjusted to the same value, and it is possible to connect the plurality of detection sections to the same electronic device.

Furthermore, the measurement capacitor C1, the correction capacitor C2, and the reference capacitor C9 in this embodiment include a cylindrical measurement ring electrode 9, a correction ring electrode 10, and a reference ring electrode 11.
and a common core electrode 12, which moves together with the spindle 15 along the central axis of these ring electrodes 9, 10, 11,
Movement in directions other than the direction of movement is restricted.

Therefore, even if the distance between the central axes changes slightly due to an impact, etc., the change in b, which constitutes the proportionality constant in equation (12), is extremely small and has almost no effect on the voltage E3. .

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

Therefore, in this embodiment, the voltages Em to Em, E.

Shield the wire connecting the detection unit and the electronic device to guide the
1 to 24 are shielded.

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

That is, as shown in FIG. 3, in the detection section, the input side of the impedance transformer 26 and one side of the discharge resistor 27 are connected to the common core electrode 12, 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 precision are not required, it becomes possible to shield the wires leading the voltages Em, E2, Em, and Em' all at once, 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 measurement ring electrode 9, the correction ring electrode 10, the reference ring electrode 11, and the common core electrode 12 of the same material, the thermal expansion of the parts becomes the same, and it is possible to prevent capacitor imbalance due to temperature changes, making it possible to detect The temperature performance of the parts can be improved.

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 reference square wave voltage E osc. 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 and 32 are electronic switches which switch between the DC voltage Er and the ground level in response to the voltage E osc to generate the reference square wave voltage E.

The electronic switch 32 switches between the DC voltage -Em and the ground level in response to the voltage E osc and outputs it as a complementary square wave voltage E2.

33 is an input amplifier that inputs the feedback voltage Em;
inputs its output voltage E4 and outputs it every half cycle of voltage EosC (tll, t12, t13"' and t old, t22, t
35 is a differential integrator that inputs the demodulated voltage, and 36 is the output DC voltage E. This is an electronic switch that switches between the voltage E osc and the ground level in response to the voltage E osc and outputs it as a measured square wave voltage E3.

In the above electronic device, the voltage E1. Since DC voltages of different polarities are intermittently output from the electronic switches 31 and 32 at the same timing, E2 becomes a square wave voltage of the same frequency and opposite phase.

Further, the voltage E3 is intermittently applied to the electronic switch 3 at the timing when the DC voltage Eo from the differential integrator 35 reaches the voltage Eosc.
It is synthesized by outputting from 6.

This DC voltage E. is synthesized as follows.

First, the feedback voltage Em is amplified by the input amplifier 33, and the amplified voltage E4 is demodulated every half cycle by the demodulator 34. Furthermore, the DC voltage E output by the differential integrator 35 as a function of the amplitude and polarity of the demodulated voltage, if the demodulated voltage is not zero, until this demodulated voltage reaches zero. is changed.

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

As a result, the voltage E3 satisfies the relationship of equation (9), is directly proportional to the displacement X, and has a positive proportionality constant.

<Effect of the invention> According to the invention, the displacement of the common core electrode 1. X is.

If the direction in which the common core electrode enters the measurement ring electrode is positive, it can be detected from the measurement square wave voltage E3 which is directly proportional to this displacement X and has a positive proportionality constant. Therefore, electronic circuits such as arithmetic circuits as in conventional examples are not required, and the influence of such electronic circuits on stability and temperature characteristics can be eliminated, improving stability and temperature characteristics. It is also possible to reduce costs.

In addition, by providing a correction capacitor and an adjustment screw for adjusting its capacitance, it is possible to adjust the zero point of the detection section without using electrical components such as a potentiometer. The influence can be eliminated, and stability and temperature performance can be improved.

In particular, since the zero point can be adjusted on the detecting section side without moving the detecting section, when used attached to machinery, it is possible to eliminate the need for a fine movement zero point adjusting device that is generally provided on the machine side. Therefore, it is particularly effective when switching and using a large number of detection units in one electronic device.

Furthermore, by providing adjustment screws for capacitance adjustment on the reference capacitor and the correction capacitor, it is possible to connect different detection units to a common electronic device without calibrating the measurement origin or the like.

Furthermore, since the measured square wave voltage Em is constituted by the ratio of capacitances, it is not affected by the dielectric constant by making the dielectrics of all capacitors the same.

Furthermore, since the center axes of the measurement ring electrode, correction ring electrode, reference ring electrode, and common core electrode are aligned, even if the distance between the center axes changes, the effect on the measurement voltage can be minimized. can.

Furthermore, by using a ball retainer in the spindle bearing and keeping it in a preloaded state, the play of the spindle can be reduced to zero, and the influence of the play of the spindle can be eliminated.

Furthermore, by providing an impedance transformer and a discharge resistor in the detection section, when high sensitivity and high precision are not required, the lines leading to the voltages E1 to E3 and Em' can be shielded together.

Moreover, the measurement ring electrode 9 that constitutes the capacitors 01 to C3
, Correction ring electric Fi! By using the same material for the lO1 reference ring electrode 11 and the common core electrode 12, it is possible to improve the temperature performance of the detection section.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the structure of a detection section of a capacitive length measuring device according to an embodiment of the present invention, and FIG. 2 shows the displacement X of the measuring core electrode and the measuring square wave voltage Em shown in FIG. FIG. 3 is a circuit diagram showing the impedance transformer attached to the detection unit shown in FIG. 1, and FIG. FIG. 5 is a time chart showing the voltage phase relationship in FIG. 4. Fig. 6 is a diagram showing the structure of the detection section of a conventional capacitive length measuring device, and Fig. 7 is a reference square wave voltage V in the length measuring device shown in Fig. 6.
, and the displacement X of the screen. 9...Measuring ring electrode, 10...Correction ring electrode, 11...Reference ring electrode, 12...Common core electrode, 15...Spindle, 16...Bearing, 18.19.42. 43...adjustment screw, 21.22.
23.24... Shield, 26... Impedance transformer, 27 ・ 30 ・ 31. 33 ・ 34 ・ 35 ・ E. E.・Discharge resistor, ・Oscillator, 32.36...electronic switch, input amplifier, ・Demodulator, ・Differential integrator, ・Reference square wave voltage, ・Complementary square wave voltage, ・Measurement square wave voltage, ・Feedback Voltage, ・Measuring capacitor, ・Compensation capacitor, ・Reference capacitor. Figure 2

Claims (4)

[Claims] (1) A cylindrical measurement ring electrode, a correction ring electrode, and a reference ring electrode arranged on the same center line, and a substantially circular shape that moves within the measurement ring electrode, correction ring electrode, and reference ring electrode along their central axes. a columnar common core electrode, and a measurement capacitor C is provided between the measurement ring electrode, the correction ring electrode, the reference ring electrode, and the common core electrode, respectively.
_1, correction capacitor C_2 and reference capacitor C_3
, and a reference square wave voltage E_ is applied to the measuring ring electrode.
1 is applied to the correction ring electrode, a complementary square wave voltage E_2 having the same frequency and opposite phase as the reference square wave voltage E_1 is applied to the correction ring electrode, and the reference square wave voltage E_1 is applied to the reference ring electrode.
When a measuring square wave voltage E_3 having the same frequency and opposite phase is applied and the displacement X of the common core electrode is zero, the capacitance c of the measuring capacitor C_1 when the displacement X is zero is
Product of _0 and absolute value of reference square wave voltage E_1 c_0 | E
_1| and capacitance c_2 of correction capacitor C2
and the absolute value of the complementary square wave voltage E_2, c_2 | E_2
| is set to be equal, and when the capacitance c_1 of the measuring capacitor C_1 changes due to the displacement of the common core electrode, a constant voltage is set by the electronic device so that the feedback voltage E_m induced in the common core electrode becomes zero. D.C.
A capacitive length measuring instrument characterized in that a measurement square wave voltage E_3 is varied by alternately switching between voltage and a variable DC voltage E_0. (2) An impedance transformer whose input side is connected to the common core electrode and whose output side is connected to the electronic device, and a discharge resistor whose one side is connected to the common core electrode and whose other side is grounded are provided. 2. The capacitance type length measuring device according to claim 1. (3) The capacitive length measuring device according to claim 1 or 2, wherein the measuring ring electrode, the correction ring electrode, the reference ring electrode, and the common core electrode are made of the same material. (4) The capacitive length measuring device according to any one of claims 1 to 3, wherein a ball retainer is used in a bearing of the spindle attached to the common core electrode to create a preloaded state.