JPH0518655Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention relates to a capacitive displacement meter that measures stroke displacement of a moving body by detecting capacitance.

(Prior Art) As a displacement meter of this type, the applicant of the present application had previously proposed a displacement meter as shown in FIG. 3 in Utility Application No. 174052/1983. In this figure, 1 indicates a cylindrical case, and 2 indicates a rod, and the stroke displacement to be measured is input as the relative displacement thereof. The rod 2 is supported concentrically within a cylindrical case 1 through an end cover 3 so as to be movable relative to each other, and a pair of inner and outer cylindrical electrodes 4 and 5 surrounding the rod 2 are further disposed within the case 1. At one end of these cylindrical electrodes 4 and 5,
The inner cylindrical electrode 4 is fixed to the cylindrical case 1 through the end cover 3 in a conductive manner, and the outer cylindrical electrode 5 is fixed to the insulator 6.
It is fixed to the case 1 in an electrically insulating manner. The electrode 4 is connected to the displacement detection circuit 8 through the end cover 3 and the case 1, and the electrode 5 is connected to the displacement detection circuit 8 through the terminal 7. A gas or liquid dielectric fluid is sealed in the space defined by the end cover 3 including the spaces 9 and 10 between the end cover 1 and the end cover 3.

A double cylinder 11 is attached to the rod 2, and this double cylinder has an inner cylinder wall 11a that enters into the gap 9 as the rod 2 strokes, and a cylinder wall 11a that enters the gap 9 as the rod 2 strokes.
It has an integral outer cylinder wall 11b that penetrates into the inside of the cylinder,
The capacitance between the ends 9 and 10 can be changed in accordance with these intrusions, that is, in accordance with the stroke displacement to be measured. Then, this capacitance is connected to circuit 8.
By detecting this, the relative displacement between the case 1 and the rod 2 can be measured.

In this configuration, the inner diameter inside the case 1 is d ₈ ,
The outer diameter of the outer electrode 5 is _d5 , the inner diameter is _d4 , the outer diameter of the inner electrode 4 is _d1 , the outer diameter of the outer cylinder wall 11b is _d7 , and the inner diameter is
d ₆ , the outer diameter of the inner cylinder wall 11a is d ₃ , the inner diameter is d ₂ ,
The maximum stroke of rod 2 is l, double cylinder 1
1, the relative permittivity of the current-permeable material between 9 and 10 is ε ₁ , and the capacitance when the cylinder 11 enters _the space between 9 and 10 by x is the capacitance.
CT can be calculated as follows.

In other words, C ₁ is the capacitance at the overlapping part between the electrodes 4 and 5 and the double cylinder 11, and the capacitance between 9 and 10 at the other part is C 1 .
C ₂ and the capacitance at the portion where the insulator 6 is present is C ₃ , CT=C ₁ +C ₂ +C ₃ (1). Therefore, if the permittivity of vacuum is ε ₀ , then C ₁ is C ₁ = 2πε ₀ x [1/(1/ε ₂ −1/ε ₁ ) _lo・d ₃ /d ₂
+1/ε ₁・_lo d ₄ /d ₁ +1/(1/ε ₂ −1/ε ₁ ) _lo・d ₇ /d ₆ +1/ε ₁・l
_o d ₈ /d ₅ ]...(2) Also, C ₂ is C ₂ =2πε ₀ (1-x) [1/1/ε _{1 lo} d ₄ /d ₁ +1
1/ε _{1 lo} d ₈ /d ₅ ]...(3), and C ₃ remains unchanged even when x changes.

By the way, in equation (2), the part connected by parentheses is a constant, so we call it A, and in equation (3), the part connected by parentheses is also a constant, so we call it B.
Then, equation (1) becomes as follows.

Cr=2πε ₀ x (A−B)+2πε ₀ B+C ₃ (4) Therefore, the capacitance CT is proportional to the stroke displacement x.

The detection circuit 8 is an RC with the above CT as a capacitor.
It is composed of an oscillator, and its oscillation period T is expressed by the following equation.

T = (1/K) RCT (K: constant) = (R/K) (2πε ₀ x (A-B) + 2πε ₀ B + C ₃
)
...(5) By the way, since 2πε ₀ B+C ₃ is constant in this equation, the amount of displacement x can be measured by reading the oscillation period T of the circuit 8.

(Problem to be solved by the invention) In such a capacitive displacement meter, as mentioned above, capacitance C ₃ is also formed at the location where the insulator 6 is present, and in order to make the above measurement highly accurate. for,
In equation (5), C ₃ must be constant. For this reason, the insulator 6 should be made of a material with a stable dielectric constant, and it is common sense to use synthetic resin such as polypropylene or polyacetal as this material.

However, this synthetic resin has a thermal expansion coefficient of case 1.
The capacitance is larger than that of electrodes 4 and 5.
The length m of the portion of the insulator 6 that is involved in C ₃ changes greatly depending on the temperature. Change in length m is capacitance
This causes a change in C ₃ and, as a result, the measurement results vary depending on the temperature, which is an unavoidable problem in terms of accuracy.

(Means for solving the problem) The present invention cannot completely eliminate the insulator because it is necessary to electrically insulate and fix the outer cylindrical electrode to the cylindrical case. The insulator is shaped so that it does not penetrate between the inner and outer cylindrical electrodes, from the viewpoint that measurement errors due to thermal expansion can be reduced by minimizing the insulator.

(Function) When the double cylinder is displaced relative to the outer cylindrical electrode, inner cylindrical electrode, and cylindrical case in response to the displacement of the moving object to be measured, these The capacitance between them changes, and the displacement of the moving body can be measured based on this capacitance change.

By the way, since the insulator used to fix the outer cylindrical electrode to the cylindrical case has a shape that does not penetrate between the inner and outer cylindrical electrodes, the part of this insulator that contributes to capacitance is reduced accordingly. Even when the amount of thermal expansion changes due to temperature changes, the influence of this on the capacitance can be reduced, and measurement errors due to temperature changes can be reduced.

(Example) Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 1 shows an embodiment of the present invention, in which the same parts as in FIG. 3 are designated by the same reference numerals.

In the present invention, in order to shape the insulator 6 so that it does not intrude between the inner and outer cylindrical electrodes 4 and 5, a recess 6 is formed in the insulator 6 so that the outer cylindrical electrode 5 is fitted into the insulator 6.
The bottom surface 6b of a is flat, and the space 9 is this bottom surface 6b.
is made flat, and the space 9 is extended to this bottom surface 6b,
This ensures that no insulator 6 exists between the cylindrical electrodes 4 and 5.

In order to limit the fitting distance of the insulator 6 to the cylindrical case 1 as shown in a, the remaining inner circumference of the cylindrical case 1 is made to have a large diameter and a step 1a is set. In this case, as shown in FIG. 2, lines of electric force as shown by the arrows tend to run at the step 1a due to the end face effect of the lines of electric force. In order to prevent this, the insulator 6 is made to overhang from the step 1a as shown by b.

The displacement meter of the present invention having such a configuration can also measure displacement in the same manner as described above with reference to FIG. Even with changes in the amount of expansion,
The influence of this on capacitance can be reduced, and measurement errors due to temperature changes can be reduced.

Further, the cylindrical case 1 and the electrodes 4 and 5 are made of metal (for example, iron), and the change in length a due to temperature changes is relatively small due to thermal expansion of the metal. Therefore, the change in capacitance formed within the range of length a is almost negligible, in combination with the fact that the dielectric constant of the insulator 6 is stable against temperature changes. On the other hand, the capacitance formed in the range of length b is larger than the capacitance between 9 and 10 because the dielectric constant of the insulator 6 is larger than that of the dielectric current between 9 and 10. The capacitance in the range of length b has a negligible effect on the overall capacitance as the temperature changes. For these reasons, the inevitable portion of the insulator 6 between the outer electrode 5 and the cylindrical case 1 does not cause measurement errors due to temperature changes.

(Effect of the invention) As described above, in the capacitive displacement meter of the present invention, the insulator 6 is shaped so that it does not penetrate between the inner and outer cylindrical electrodes 4 and 5, so the insulator 6 has a correspondingly low capacitance. By reducing the number of parts involved, the influence of changes in the amount of thermal expansion due to temperature changes on capacitance can be reduced, and measurement errors due to temperature changes can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing one embodiment of the displacement meter of the present invention, FIG. 2 is a partially enlarged sectional view thereof, and FIG. 3 is a sectional view of a displacement meter previously proposed by the applicant. 1... Cylindrical case, 2... Rod, 3... End cover, 4... Inner cylindrical electrode, 5... Outer cylindrical electrode,
6... Insulator, 7... Terminal, 8... Displacement detection circuit, 9, 10... Between, 11... Double cylinder.

Claims (1)

[Claims for Utility Model Registration] A pair of concentrically arranged cylindrical electrodes are provided concentrically within a cylindrical case, and at one end of these electrodes, the inner cylindrical electrode is fixed to the cylindrical case in a conductive relationship, and the outer cylindrical electrode is fixed to the cylindrical case in a conductive relationship. A cylindrical electrode is fixed to the cylindrical case via an insulator in an electrically insulating relationship, and the relative displacement of the double cylinder within the gap between the outer cylindrical electrode, the inner cylindrical electrode, and the cylindrical case is controlled. The capacitive displacement meter measures by changing the capacitance between the two, wherein the insulator has a shape that does not penetrate between the inner and outer cylindrical electrodes. .