SU1767451A1 - Metering transducer with capacity sensor - Google Patents

Abstract

The invention relates to the technique of measuring non-electrical quantities by electrical methods and can be used to measure small displacements, the level of substances, their humidity, etc. The transducer with a capacitive sensor contains a source 1 of constant stable voltage, the first 2, second 3 and third 4 switches, first 5 and second 7 exemplary capacitors, measuring capacitive sensor 6, charge-to-voltage converter 8, 9 rectangular pulse generator, discrete integrator 10, inverting amplifier II Providing a partial compensation of the discharge current of the measuring sensor increases the sensitivity of the conversion and allows you to extend the functionality of the converter 1 cnffly 2 il

Description

The invention relates to the technique of measuring non-electrical quantities by electrical methods, is intended to convert the parameters of a capacitive sensor into a change in electrical voltage and can be used to measure small displacements, the level of substances, their humidity, etc.

A non-electrolytic measuring transducer with a capacitive sensor is known, which comprises an alternating voltage generator, a measuring and exemplary capacitors, and an operational amplifier. A significant disadvantage of the converter is its low sensitivity during operation -. capacitive sensors having a large initial capacitance and a small value of its increment caused by a change in the monitored parameter. An additional disadvantage is the dependence of the output voltage of the converter on the instability of the voltage of the sinusoidal voltage generator.

A measuring transducer with a capacitive sensor is known, which contains a source of stable constant voltage, two switches, working and exemplary capacitors, an imbalance signal amplifier, a phase-sensitive detector, a voltage divider, a low-pass filter, an additional amplifier, an adder and a square generator pulses. The disadvantage of such a converter is low sensitivity to a low-level informative low-frequency component.

A capacitor-to-voltage converter is also known, which contains a source of constant stable voltage, a capacitive sensor and an exemplary capacitor, two switches, a discrete integrator and a square pulse generator. The converter has a non-linear dependence of the output voltage on a change in the capacitance of the sensor and is intended primarily for operation with sensors having a hyperbolic dependence of the capacitance on the measured value. The disadvantage of the device is low sensitivity when working with a capacitive sensor with a large initial capacitance and a small increment of capacitance due to a change in the monitored parameter.

The closest technical solution is a capacitance-to-voltage converter containing two sources of constant stable voltage, a capacitive sensor, two model capacitors, three switches, and a converter.

charge voltage and square pulse generator.

The converter has a linear dependence of the output voltage on the change in the capacitance of the sensor. When used with sensors that have a hyperbolic dependence of capacitance on the measured value, it is advisable to change the switching point of the capacitive sensor, which allows to obtain a linear dependence of the output voltage of the converter on the measured value.

In both cases, when the sensor is turned on, the converter has a low sensitivity when working with sensors that have a large initial capacitance and a small increment from the change in the monitored parameter.

The purpose of the invention is to increase the sensitivity of the converter when working with capacitive sensors having a large initial capacitance with a small increment due to a change in the monitored parameter.

The goal is achieved by providing a partial compensation of the charge of the capacitive sensor at the input of the converter, charge to voltage, for which the measuring converter of capacity to voltage, containing a source of constant stable voltage, the first, second and third switches, the first and second capacitors, measuring capacitive sensor, charge-voltage converter, discrete integrator and square pulse generator, an inverting amplifier is inputted, the input of which is connected to the output the transmitter, and the output is connected to the first information input of the third switch.

FIG. 1 shows a block diagram of a capacitance-voltage converter; and in fig. Figure 2 shows the output voltage of the converter for various values of sensor capacitance.

The converter contains a source of constant voltage 1, switches 2, 3 and 4, first and second capacitors 5 and 7, measuring capacitive sensor 6, charge-to-voltage converter 8, square-wave generator 9, discrete integrator 10 inverting amplifier 11. The current electrodes of both exemplary capacitors and the measuring capacitive sensor are connected together and connected via a charge converter 8 to a discrete integrator 10. A constant stable voltage source 1 is connected to

The first information input of the first switch 2 is connected to the potential electrode of the reference capacitor 5. The output of the discrete integrator 10 is connected to the first information input of the second switch 3 and the input of the inverting amplifier 10, and the output of the second switch is the 3rd potential electrode of the capacitive sensor 6.

The first information input of the third switch A is connected to the output of the inverting amplifier 11, and the output of this switch is connected to the potential electrode of the second exemplary capacitor. The second information inputs of all switches are connected to the common bus device. The output of the generator 9 rectangular pulses is connected to the control inputs of the switches and the discrete integrator.

The device works as follows.

The generator 9 of rectangular pulses controls the operation of the switches 2, 3 and 4. The potential electrodes of the reference capacitor 5 and the measuring capacitive sensor 6 are supplied by rectangular pulses of opposite polarity, formed by the switch 2 from the voltage Uo of the source 1 of a constant stable voltage and the switch 3 from the output voltage of the discrete integrator 10. The potential electrode of the second exemplary capacitor 7 receives rectangular pulses generated by the switch 4 from the output voltage of the inverter A rotary amplifier 11, to the input of which a voltage is supplied from a discrete integrator 10. Square-wave pulses periodically charge and discharge both exemplary capacitors and a capacitive sensor. The input current of converter 8, proportional to the algebraic sum of these charges, is converted into voltage pulses that accumulate in the discrete integrator 10.

A change in the capacitance of the measuring sensor, caused by a change in the monitored parameter, leads to an imbalance of the charges at the input of the charge converter, which is compensated for by changing the output voltage. 11 Out of discrete integrator 10 , Su and capacitive sensor Ce is zero.

UoC5 + UBbixC6-KUBb, xC7 0, (1)

where Cs, С are electrical capacitances of model capacitors,

Ce is the current value of the capacitance of the sensor, depending on the measured non-electrical value;

K is the transmission coefficient of the inverting amplifier.

The transfer function of the device is

UoCs

0

U

out

Sat - K C

(2)

five

0

five

0

five

The dependences of the output voltage of the device on the change in the capacitance of the sensor at two different values of the charge on the reference capacitor Cs are shown in FIG. 2. As can be seen from the above characteristics, with large values of the sensor capacitance, the dependence of the output of the output f (c) has a small slope (section CD in FIG. 2), which leads to a low sensitivity of the device

When using the proposed technical solution, it is possible to shift the section of the CD characteristic to the region of greater conversion steepness (section AB). As follows from formula (2), this is achieved by selecting the appropriate value of the second exemplary capacitor C and (or) transmission coefficient K of the inverting amplifier 11.

The resulting increase in sensitivity can be shown as follows.

In the absence of an inverting amplifier

UoCs

a AUE

UE

 -UoCs

40

Sensitivity Si Converter

A TS.yy Uo Cs

ASHZ Sh4

After the introduction of the inverting amplifier U0C5

willow2 -

eleven

; DIV2 UoCsx

50

 - C 7 CX3 - C

transducer sensitivity in this case

55

/

  A Ubix2Uo C5

2 (Dx4 - C7) (SkH S)

Therefore, 52 SL. The indicated increase in sensitivity cannot be obtained by simply increasing UoCs, which is shown goofically in FIG. 2 curve 2.

Thus, the proposed technical solution makes it possible to increase the sensitivity of the converter and use it with sensors having a large initial capacitance, which further extends its functionality.

Claims (1)

The invention includes a measuring transducer with a capacitive sensor, containing a source of constant stable voltage, first, second and third switches, measuring capacitive sensor, first and second exemplary capacitors, charge-voltage converter, discrete integrator, square pulse generator, and elements of the first and second exemplary capacitors and measuring capacitive sensor are connected and connected via a charge-voltage converter to the input of a discrete integrat pa, potential electrodes O & Shg C the first exemplary capacitor, measuring capacitive sensor and the second exemplary capacitor are connected respectively to the outputs of the first, second and the third switch, the first information input of the first switch is connected to a constant stable voltage source, the first information input of the second switch is connected to the output a discrete integrator whose output is the output of the converter, the second information inputs of the first, second and third switches are connected to the common bus of the measuring converter, and the output of the square pulse generator is connected to the control inputs of the three switches and the discrete integrator, characterized in that increase the sensitivity of the converter; an inverting amplifier is introduced into it; its input is connected to the output of the discrete integrator, and the output of the inverting amplifier is connected chen to the first information entry of the third switch. ЈХ4 # &. J