SU1226015A1 - Differential variable-capacitance linear displacement transducer - Google Patents

Abstract

The invention relates to a measurement technique and can be used to control linear displacements in conjunction with an autogenerator secondary transducer and improves the linearity of a linear capacitance-type transducer type conversion function containing two pairs of opposing electrodes differing in area by no less than 4-7. times, and a grounded shield disposed therebetween, mounted to move towards them. Each electrode of one pair is connected through one of the coupling capacitors to the electrode of the other pair located on the other side of the screen. The distance between the electrodes of the second pair is 10-25% smaller than the gap between the electrodes of the first pair, and the screen sizes exceed the dimensions of both pairs of electrodes, which eliminates edge effects. When moving the screen associated with the test object during the measurement, the distance between it and the sensor electrode pairs changes, which causes a change in the frequencies of the two autogenerators, and hence the difference frequency, which changes in proportion to the controlled movement due to the selected ratios sizes of electrodes and sizes of capacitors. 2 Il. se bO N3

Description

The invention relates to a measurement technique and can be used to control linear movements in conjunction with an autogenerator secondary transducer.

The purpose of the invention is to increase the linearity of the transform characteristic of the sensor.

FIG. 1 shows a structural diagram of the sensor; in fig. 2 (a, b, c) - of

capacitance between electrodes and ec

sensor wound, changing the frequency f: and 2 at the outputs of the oscillators and the difference frequency Af at the mixer output, respectively.

The sensor contains the first pair of flat electrodes 1 connected to its outputs and the second pair of electrodes 2 installed next to them, the area of which is 4-7 times the area of the electrodes of the first pair. Between them is placed a shield 3 connected to a common grounding point. Between each electrode 1 of the first pair and lying on the other side of the screen electrode 2 of the other pair are included capacitors 4. The electrodes 2 of the second pair are set at a distance between themselves that is shorter (gap) between the electrodes 1 of the first pair by the value of b. Screen 3 is made by crossing the area of both pairs of electrodes, for which its dimensions are larger than the total dimensions of these electrodes. Screen 3 is installed in the gap between electrodes 1 and 2 with the possibility of moving X towards them. To the output terminals 5 of the sensor, to which electrodes 1 of the first pair are directly connected and through capacitors 4 — electrodes 2 of the second pair, are connected two auto-oscillators 6 and 7 and a frequency comparison unit that separates the difference frequency and is made in the form of a mixer 8.

The differential capacitive linear displacement sensor operates as follows.

With a symmetrical position of the screen 3 with respect to the electrodes 1 and 2, the capacitance values between the output terminals 5 of the sensor and its common point are equal to each other, which corresponds to a zero output signal of the sensor. When screen 3 is moved from the position of symmetry, for example, to the right, by the value of X, the capacitance Ci between it and the nearest electrode 2 increases according to a hyperbolic law.

C, -i. (I)

However, since the electrodes 2 are connected to the output terminals 5 of the sensor via capacitors 4, the equivalent change in capacitance C due to movement X is

2)

five

0

0 ..

0

0

five

five

where b is a constant determined by the value of the capacitance of the capacitors 4.

The capacitance value of € 2 between screen 3 and near electrode 1 also increases as screen 3 moves to the right. Due to the fact that the electrodes 1 are located farther from the screen 3, the dependence of this capacitance on the magnitude of the displacement X is

 ; (3)

where b is the difference value of the gaps between pairs of electrodes 1 and 2.

The total value of the indicated sensor capacitances is also influenced by the capacitance between electrodes 1 and 2, from which screen 3 moves away when X is moved. However, the influence of these capacitors on the change in the total capacitance of the sensor at the boundaries of the measurement range can be neglected due to their smallness. Regarding the values of capacitances C: (2) and Ca (3), measured with the help of autogenerators 6 and 7 and the mixer 8, determine the value X of the displacement of the screen 3 and the associated control object. Dividing (2) by (3), we get:

(four)

Thus, the magnitude of the displacement X is determined by the following expression:

Vop. 1 / g

X-Yjjip (5)

Since the constants b and b are free parameters, they can be chosen so that the fractional term in (5) does not depend on X. Indeed, the value

Bx + 1 (bx + l) b.

-xTF- bx + bb provided that.

Based on the practically existing requirements for maintaining the sensitivity of the sensor and the range of controlling movements, it can be shown that this condition is satisfied if the area of the second pair of electrodes is 4-7 times larger than the area of the electrodes of the first pair, the distance between them is 10-25 % less than the gap between the electrodes of the first pair, and the capacitance of the capacitors 4 - 10-20 times greater than the nominal (in the initial position) capacitance of the sensor.

Since the fractional term of expression (5) is practically unchanged at the indicated ratios, the transform characteristic of the sensor is close to linear.

Claims (1)

Invention Formula Differential capacitive linear displacement sensor containing a pair electrodes installed with a gap opposite one another and connected to the output terminals of the sensor, and a screen placed parallel to them in the gap, capable of moving in the direction of the electrodes and connected to the test object during the measurement process, characterized in that in order to increase linearity conversion characteristics, it is provided with a second pair of electrodes, the area of which exceeds the area of the electrodes of the first pair and which are installed opposite each other on opposite sides of the screen at a reduced distance between each other, compared with the gap between the electrodes of the first pair, and the two capacitors connecting each electrode of one pair to the other pair located on the other side of the screen, and the screen sizes exceed the total dimensions of both pairs of electrodes. .J Compiled by L. Kislntsin Editor N. TupitsaTehred I. VeresKorrektor O. Lugova Zakaz 1978/26 Circulation 670 Subscription VNIIPI USSR State Committee for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab. 4/5 Branch PPP "Patent, Uzhgorod, st. Project, 4