SU1631308A1 - Torsion balance - Google Patents

Abstract

The invention relates to devices for measuring small forces, namely, torsional weights, and can be used in scientific research when measuring small forces acting on various objects in electric magnetic and gravitational fields. Torsion scales containing a rocker suspended from a support on the torsion bar and a device for measuring the angle of the pair, which is made in the form of two capacitive displacement sensors, each of which consists of two parallel-connected and symmetrically located relative to the axis of rotation of the capacitors, the movable plates of which are fixed on the yoke . The principle of the scale is based on measuring the capacitance of the capacitors of the first and second sensors. By the difference of their testimony, the magnitude of the angle of the rocker arm and the force acting on its shoulder is judged. 3 il.

Description

The invention relates to devices for measuring small forces, namely, torsional weights, and can be used in scientific research to measure small forces acting on various objects in electric, magnetic and gravitational fields.

The purpose of the invention is to improve the accuracy of the scales.

On fig.Ch depicted kinemag. "SSC scheme; Fig. 2 is an electrical block diagram of a torsional scale; Fig. 3 shows the principle of measuring the angular displacement of the rocker arm when a horizontal force is applied to the specimen and at the same time flat displacement of the rocker arm, top view of moving and stationary plates of capacitive sensors.

The essence of the invention is as follows. Since each capacitive sensor consists of two parallel capacitors, the fixed plates of which, belonging to the same capacitive sensor, are located on opposite sides of the vertical plane passing through the movable plates of this sensor, fixed on different arms of the rocker arm, then with a flat displacement of the entire rocker arm by d, (FIG. 3) the distance between a pair of plates of one capacitor increases by d and its capacitance decreases, while the distance between a pair of plates of the second capacitor included in the same bone sensor decreases by b and its capacity increases. In case of equality of the original capacitances, both are connected in parallel about

00

capacitors of the sensor, which is achieved by adjusting the distance between the plates, the capacitance value of this sensor MF (which is equal to the sum of the capacitances of the two capacitors) is related to the angle of rotation at a small angle a (Fig. 3) and simultaneous flat offset by distance of ± 6 (5 “Di) as follows

Cidi (1.1

-I- oL ±

WITH

 (one

oL, b2lgp

-3T + rf)

where C ° is the capacitance value of the first sensor when the source of external force is off,

di is the distance between the movable and fixed plates of the capacitors included in this capacitive sensor, with the external force source turned off;

a is the angle of torsion of the rocker arm;

L-shoulder length of the rocker arm.

Since two capacitive sensors are used to register the displacement, the fixed plates belonging to different sensors, mounted on the same arm of the rocker arm, are fixed on opposite sides of the vertical plane passing through the movable plates of the same sensor, when the beam is rotated through an angle a , as a result of which the capacitance of the first sensor decreases (1), the capacitance Ca of the second sensor increases, as the distance between its movable and fixed plates decreases by a L (Fig. 3). Therefore, when the rocker is rotated through an angle a and a simultaneous flat displacement of ± b (d «da), the capacity value of this sensor is equal to

G9 rb11 °;

/ -, 202 / -I, I - yy.-J-

022 ™ (d2-aL ± d + d2 - aL ± 3 -2 L d C ° 2 + MF

(d -620) С ° 2 -С ° 1

+

(rt dr

(3)

0

When adjusting the distances di and da, which is done when the external force is turned off (i.e., at), the readings of the AC signal subtractor unit are equal to zero, therefore it follows from equation (3) that:

(4% (4) d2 dt

Substituting equality (4) into equation (3), we obtain

(f + §) + + f-f) (# - d8) (5)

In equality (which is achieved by preliminary adjustment of the sensors after the sample is fixed on the beam and the latter is relaxed in the equilibrium position), expression (5) can be written as

22

AC (C ° 2 + C ° 1) a + .dod) xy

X

с ° 2-s

| C ° 2 + C ° 1

(6)

Thus, the second term in square brackets, which is proportional to the fluctuation plane displacement, contains a small factor as the coefficient of proportionality, which for d «d:

C ° 2 + MF

С ° 2-С ° 1 С ° 2-С

CO g 2 + C "1

, go (I 2 | L,

- about 2 V (2)

d2 d2

where Сa is the capacitance value of the second sensor when the source of external force is off;

d2 is the distance between movable and fixed plates of capacitors included in this sensor, with the external force source turned off.

Since the signal subtractor calculates the difference in the capacitance values of the two sensors, the result of the subtraction A C is related to the values Ci and C2 as follows

AU Сё Ci - С§

(f + $) V

(7)

The second part of the inequality is accomplished by adjusting the displacement sensors, in

the result of which C ° 2 - C ° tends to zero.

As a result, with a flat displacement of the rocker, the readings of both sensors mutually compensate each other, which leads to an effective suppression of the contribution of flat oscillations and a significant increase in the sensitivity of flat weights to torsional displacements. Negrebral the term containing the fluctuation displacement ± d, from what

the desired angle of rotation of the rocker arm will be equal (at C ° 1 C ° 2)

(eight)

The balance consists of a fixed platform 1 with a bracket 2, to which, using a torsion bar 3, the beam 4 is freely suspended, equipped with a system 5 for fastening the sample 6 in the zone of action of an external force created, for example, by an electromagnet 7. On the yoke on either side of its axis rotation is vertically fixed and electrically interconnected by four grounded metal plates 8-11, which are movable plates of two capacitive sensors. The respective fixed plates 12-15 of the same sensors are fixed on platform 1, and the fixed plates 13-15 are equipped with devices 16-18 for adjusting the distance between the fixed plates 13-15 and the movable 9-11, respectively. Thus, the first capacitive bias sensor includes two capacitors formed by plates 8 and 12, 10 and 14, and the second one - plates 9 and 13, 11 and 15. The scales are equipped with a switching device with switches 19-22 for connecting stationary sensor plates to the device 23 alternately connecting said plates to the input of the capacitance measuring unit 24. The output of the block 24 is connected to the input of the subtracting unit 25. Its output is connected to the recording system 26. The clock generator 27 is connected to the control inputs of the device 23 and the block 25. In the operating n mode Switches 19 and 21 are closed, so capacitors entering the same capacitive sensor are connected in parallel.

Scales work as follows.

In the initial state, the source of external force (electromagnet) 7 is turned off. After securing sample 6 on the yoke 4 and calming the latter in the equilibrium position shown in Fig. 3 with a solid line, the fixed plates 12-15 are alternately connected using switches 19-22 and device 23 to block 24, thereby measuring the value of each capacitance of the four capacitors formed by the fixed plates listed above and the movable plates 8-11, respectively. By means of adjusting devices 16-18, the plates 13-15 are so positioned that the capacities of all four of the indicated capacitors are equal. Thereafter, the alternating device 23

0

0

five

five

0

five

0

five

shifted to a position in which, by signals from the clock generator 27, it alternately connects one or the other capacitive displacement sensor to block 24. In this case, the fixed plates of each sensor are electrically interconnected. At the same time, from the signals from the generator 27, the subtraction unit 25 calculates the value of the differential capacitance of the two capacitive sensors. As a result, at the end of each switching cycle of the capacitive sensors and subtraction unit 25, the recording system 26 registers a zero signal, since the capacities of the displacement sensors with the external force source turned off are equal to each other With 1 ° C 2. Then turn on the source of external force 7, which acts in the direction indicated in Fig. 3 by the arrow on the trimmer 6 and causes the rocker arm 4 to twist on the torsion 3 at an angle “(the dotted line in FIG. 3 is a new equilibrium position of the rocker arm) proportional to the external force. This twisting leads to such a change in the capacitances of the displacement sensors, that the capacitance of one sensor increases by D C / 2, and the other decreases by the same amount, therefore the difference between the capacities of the two displacement sensors amounts to AC. As a result of subtracting the signals from the displacement sensors, a non-zero signal A C (uniquely related to the angle of rotation C) recorded by the recording system 26 appears at the output of the block 25. Since the L values are known device constants, the angle of torsion of the rocker caused by the external force and calculated by the measured

the values of A C and C ° C 2 with the expression (8). The magnitude of the external force acting on the sample is judged by the coefficient of proportionality between the external force and the angle of torsion of the rocker arm, which is determined by calibration measurements and is a constant of the instrument.

Claims (1)

The invention of torsional scales, containing a rocker arm suspended from a support on a torsion bar and a twist angle measurement device, characterized in that, in order to increase accuracy, the twist angle measurement device is made in the form of two capacitive displacement sensors, each of which consists of two parallel-connected and symmetrically located relative to the axis of rotation of the rocker arm of the capacitors, the vertical movable plates of which are fixed on the rocker arm, and the vertical fixed plates are connected with the input of the capacitance measuring unit, the output of which is connected to the first input of the subtraction unit, the output of which is connected to the input of the recording system, and the second 2 the input is with the first output of the clock generator, the second output of which is connected to the home device of the alternate connection of the plates.