JPS5821502A - Electrostatic capacity type displacement detecting circuit - Google Patents

Abstract

PURPOSE:To obtain high detecting sensitivity by controlling an AC which is flowed through a displacement converting capacitor at a constant value, and making the power source voltage of an oscillator proportional to the displacement. CONSTITUTION:A voltage vp is applied to the displacement conversion capacitor 12 from an oscillating coil 11b of an oscillator 11. A voltage vd across a current detecting resistor 13 is rectified 14, and a DC voltage Vd is obtained. Said voltage Vd is compared 16 with a reference voltage Ec. The resulting outout voltage Vs is supplied to the oscillator 11 as a power source. A DC I, which is obtained by converting 17 the voltage into the current, is supplied to a circuit 14, a comparator 16, and a circuit 17 through a constant current circuit 18. In this constitution, since the power source voltage Vs of the oscillator 11 is proportional to the output AC voltage vp of the oscillator, the displacement can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a capacitive displacement detection circuit that converts one mechanical displacement into a change in capacitance and detects it in the form of an electric signal, and particularly for one mechanical displacement. This allows a stable and linearly proportional detection output signal to be obtained.

Generally, this type of capacitive displacement conversion circuit is required to have a highly linear relationship between the mechanical displacement and the detection circuit output. However, in the conventional displacement conversion circuit of this type, the trap of converting mechanical displacement Δ into a change in capacitance of 0 is that the movable electrode I moves by the mechanical displacement Δ and the fixed electrode It is common to use a displacement conversion capacitor in which the fixed electrodes of the capacitor are arranged facing each other, and the capacitors are drawn out by a lead g) through an insulator J. is obtained.

Here, -1 dielectric constant d: Initial value of inter-electrode distance Mu, electrode area 0: Initial value of capacitance, and the capacitance 0 of the displacement conversion capacitor in the conventional displacement detection circuit of this type is It can be seen that it is not linearly proportional to the mechanical displacement Δ.

FIG. J shows a conventional circuit configuration in which a displacement detection output signal linearly proportional to the mechanical displacement Δ can be obtained using such a displacement conversion capacitor.

The circuit configuration shown in the figure is for displacement conversion = nden fO□
An oscillator with the comparison content 'y''0@ as the frequency determining element! and the oscillation frequencies f1 and f of B
The oscillation outputs having each of ■ are led to the mixer 7, and the frequency ! ! ! 1~! The mixed output containing snow is introduced to a one-frequency discriminator t via a low-pass filter t, and a displacement detection output voltage V is obtained. Namely.

The frequency difference If between the oscillation frequencies ft and fs is as follows.

So, when Δ7a<i, The displacement detection output voltage V is.

As can be seen from this equation (5), 1 mechanical leopard displacement /
A displacement detection output voltage EEV proportional to IK was obtained.

However, in the conventional circuit configuration described above, if one mechanical displacement l is relatively large with respect to the initial value d of the distance between the electrodes, it may be due to deviation from linear proportionality or Kli as shown in the following table. It had the disadvantage of being large.

That is, Δ/d is usually X~m-, and the deviation at that time is /, see~! It becomes extremely large at 7%. therefore,
In order to obtain a good direct proportional relationship, it is necessary to suppress the damage to a few strokes or less, and since the detection sensitivity Δf/fo is approximately equal to Δ/d, it is necessary to obtain a large detection sensitivity. Became.

Furthermore, in the conventional circuit configuration described above.

Another drawback was that the influence of stray capacitance was large. In other words, the above-mentioned equation (3) is such that the displacement conversion capacitance 0· naturally includes a stray capacitance Os, and the influence of the stray capacitance Os can be ignored.

becomes. Therefore, the ratio of *S capacitance O1 to electrostatic capacitance 0°, which is 0.10°, is generally a/~aI in practice II, so that the stray capacitance is 0. is the detection sensitivity Δ! /10v, further deteriorating the direct-weight proportional relationship.The conventional circuit configuration described above also has the disadvantage that it is difficult to compensate for the influence of stray capacitance.

Furthermore, the conventional circuit configuration described above also has the drawback of instability of a single oscillation frequency. Namely.

The stability of the LO oscillator is generally about 10" (/stroke), which is significantly higher than the detection sensitivity I~XJ%flC, which causes a drift in the zero point of the displacement detection output voltage and the sensitivity, which is a major drawback. Become.

An object of the present invention is to provide a capacitive displacement detection circuit that eliminates the various drawbacks of the conventional methods described above at once, maintains a good direct proportional relationship, and stably obtains high detection sensitivity. It's about doing.

That is, the capacitive displacement detection circuit of the present invention is as follows.

A capacitor whose capacitance changes according to mechanical displacement and a resistor are connected in series and the oscillation output voltage of the oscillator is applied.
The AC voltage appearing across the tNN low resistance is rectified and converted to a DC voltage, and the output voltage of the comparator circuit that compares the DC voltage and the reference voltage is the output voltage of the comparison circuit. The DC voltage is maintained equal to the reference voltage by supplying the DC voltage to the reference voltage, and the output voltage of the comparator circuit is used as a displacement detection output.

The present invention will be described in detail below with reference to the drawings.

An example of the principle circuit configuration of the displacement detection circuit of the present invention is shown in FIG. 3, and one embodiment thereof is shown in FIG. In the illustrated configuration example, the oscillator /l is an extremely stable oscillator configured with a mechanical resonator //a such as a crystal or ceramic resonator, an oscillation coil //b, and an active element that drives these elements. It has a high oscillation frequency. Among them, one oscillation coil l/b has a displacement conversion capacitor (0,) /2
Connect the current detection resistor (Ra) /J and t,'C
The AC voltage 14 at both ends due to the current detected by the resistor (R4)/JK is rectified by the rectifier circuit l# to form the DC voltage Vd. Warping DC current EEV4 and reference voltage FE<me)
ts is compared with the comparator /4K, and the comparison output voltage V, is supplied to the 1 oscillator // as the power supply voltage, and the comparison output voltage V,
is converted into a DC current l proportional to
7 and their circuit elements #, /4. /7 is operated with a single current.

Note that /9m, /9%, 11m, and /lri are terminals, respectively. DC power supply 1. and the load is in series I
I! Ribbed.

It consists of two-wire displacement detection aimmt.

To explain the operating principle of the displacement detection circuit of the present invention with the above-mentioned m-path configuration, the capacitance for displacement conversion is 0°.

Q, -Q, @ , , /d
(a) This capacitance is 0. frequency! The AC voltage Wp acting on the AC current jll lp k
t-Ra/ Jglo, (/ Because it can be done.

14-Js/fp @Os (b
), the above-mentioned resistance (Ill)/JK appears and the alternating current E! As for the mat.

v4 = R4-a4■Jgj fp ・o, ・R4(
e), and the DC voltage v4 from the rectifier g circuit/# is.

vd 1 to 1,000 - R4 (') becomes. Here, ski is a proportionality constant.

On the other hand, 1 oscillator/l is in the form of a Hartley circuit or a Lubitz circuit, and its power supply voltage V and oscillation output AC voltage are 1 oscillation starting voltage V as shown in 1g.
They have a proportional relationship with each other in the power supply voltage range higher than o. Gakatsu C.

V, Tsuksay, （・）
In particular, *ks is the proportionality constant. Also.

Comparison output voltage FEv of comparator/4 is supplied to 1 oscillation@//, 1 oscillation coil 11b, current/detection resistor (R4)/
J & rectifier rectifier circuit l valve input voltage EEVa of comparator 14
So, for the reference voltage 1@, C1 R5-V4 (f)
It is controlled so that the following relationship holds true. Shitakatsu [.

v4=a+JKf*17. @Q, *IL
d-J sea 31゜This formula can be rearranged for an AC voltage of 1TpK.

Substituting this (transformation) 2K (a) formula:

However, in this equation (1), πs kl* k3.
Let R4゜0゜ be a turtle constant.

V, test-no/l
(j), and the comparative output voltage 'fE V
It can be seen that the change in is proportional to 1Δ/dK.

This comparison output voltage M is converted into DC current IK by the voltage-current conversion circuit n.

Here, x8°mat is a constant current supplied to each circuit element 4.7 by the constant current circuit/1, and is unrelated to the displacement Δ. Therefore, in the range where the displacement detection output current I of the two-wire displacement detection circuit is ~20 m^, the output current value am^ when the displacement input is zero is this constant current I. This corresponds to nst.

Next, regarding the voltage-current conversion circuit 77 in the detailed configuration shown in figure J, the reference voltage (1,) will be explained in more detail.
/j, the following equation holds between the voltage across the zero adjustment resistor - and the voltage ve across the span adjustment resistor R1 in the conversion circuit n.

On the other hand, the comparison output voltage V is obtained from equation 0).

vs = V,. (l-Δ/d)
). However, V:!・Is it controlled to zero? Because we are doing the adjustment.

therefore.

However, since it is V・MaRl・I.

This equation (q) provides an output current I proportional to the mechanical displacement jK.

Next, the mode of compensation for the influence when the stray capacitance C1 exists will be described. In this case, the displacement conversion capacitance C5 is expressed by the following equation.

0, = 00 ・+ Os (r)l−
Δ/4 Therefore, as shown in Fig. 1, coils with the same number of windings are connected to each other in opposite phases, and a capacitor O for compensating stray capacitance is connected.
e When connected, the voltage 9 across the current detection resistor R4
d is.

, where 0. -Oc, then 0) formula becomes (d)
It becomes similar to the formula, and the stray capacitance is 0. The influence of can be completely removed.

Figure 5 is a circuit diagram of another embodiment of the present invention.

In Figure 1, parts having the same functions as those in the embodiment shown in Figure 1m7 are given the same reference numerals. #I In the embodiment shown in Figure 7, constant current circuits /I are used, but according to the embodiment shown in Figure 1, only one constant current circuit /I is required. Note that in the V/I conversion l)n, Rf is a feedback resistor used at in the two-wire displacement detection circuit.

As is clear from the above description, according to the present invention.

For displacement conversion, note that the power supply voltage of the second oscillator and the oscillation output voltage are proportional; since the displacement detection circuit is constructed by controlling the flowing alternating current to a constant value, An effect is obtained in which the voltage is proportional to the displacement, and therefore a displacement detection characteristic exhibiting extremely good linearity is obtained.

Furthermore, since the oscillation Ils using a mechanical vibrator with an extremely stable oscillation frequency is used, an extremely stable displacement detection output can be obtained, and the influence of external conditions such as temperature can be extremely reduced.

Note that the displacement detection circuit of the present invention has a seventh structure in which a fixed electrode is arranged opposite to a diaphragm used as a pressure-receiving element.
The same effect as described above can be obtained by using a pressure detector using a displacement conversion capacitor configured as shown in the figure.

[Brief explanation of the drawings] Figure 1 is a diagram showing an example of the configuration of a displacement conversion capacitor;
Figure 2 is a Prutske diagram showing the configuration of a conventional displacement detection circuit, Figure J is a Bootske diagram showing the basic configuration of the displacement detection circuit of the present invention, and Figure 2 is a circuit diagram of an embodiment of the present invention. ,Horse!
4 is a diagram showing an example of the characteristics of the oscillator, FIG. 4 is a circuit diagram showing an example of the configuration of the oscillation circuit, and FIG. 7 is a diagram showing an example of the oscillation circuit. l...movable electrode, - one fixed electrode. J...Insulator, -...Lead! I. j, 6... oscillator, 7... mixer. L...Low pass filter, T...Voltage-current conversion circuit. /c...oscillator, //a...mechanical oscillator. //b...Oscillation coil, '/condenser for displacement conversion. IJ...Current detection resistor, /#-- Rectifier circuit. IJ...Reference voltage, /4-...Comparison Wh. 17...Voltage-current conversion circuit, /I-... Constant current circuit. 19...External terminal, 〃...Zener diode. Patent applicant: Fuji Electric Manufacturing Co., Ltd. Figure 1 Figure 2

Claims (1)

[Claims] A capacitor whose capacitance changes according to mechanical displacement and a resistor are connected in series and the oscillation output voltage of the oscillator is applied.
The AC voltage appearing across the resistor is rectified and converted into a DC voltage, and the DC voltage is made equal to the reference voltage by supplying the output voltage of a comparison circuit that compares the DC voltage and a reference voltage to the oscillator. A capacitance type displacement detection circuit, wherein the output voltage of the comparison circuit is used as a displacement detection output.