SU1718145A1 - Capacitive sensor based transducer - Google Patents

Abstract

The invention relates to the technique of measurement. This method is used to convert the parameters of a capacitive sensor to a change in the output voltage. The aim of the invention is to expand the range of converting capacitance to voltage. The measuring transducer with a capacitive sensor contains the first constant voltage source 1, the first 3 and second 4 switches, the measuring capacitive sensor 6, the first exemplary capacitor 7, the charge-to-voltage converter 9, the discrete integrator 10, the generator 11 square-wave pulses. The introduction of a second constant voltage source 2, the third switch 5 and the second exemplary capacitor 8 partially compensates for the charge of the capacitive sensor at the input of the charge converter, which significantly expands the measuring range of the sensor capacitance. 2 Il.

Description

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

A known converter using a close inductive coupling bridge, which contains a stabilized amplitude sinusoidal voltage generator, operating and exemplary capacitors, a constant sinusoidal voltage converter, an amplifier, a phase-sensitive detector.

A disadvantage of the known device is its complexity and low-tech production due to the presence of a large number of winding elements.

A measuring transducer of meelectric values with a capacitive sensor is known, comprising an alternating voltage generator, a measuring and exemplary capacitors, and an operational amplifier.

The lack of a converter is low conversion accuracy due to the instability of the sinusoidal voltage generator.

A non-electrical measuring transducer with a capacitive sensor is known, which contains a source of constant stable voltage, operating and exemplary capacitors, two switches, an amplifier, a phase-sensitive detector, and a square-wave generator.

The disadvantage of the converter is the small measuring range, limited by the level of the output voltage corresponding to the initial value of the capacitance of the sensor, and the maximum allowable output voltage of the device.

The closest in technical essence to the present invention is a capacitance-voltage converter containing a source of a constant stable voltage, a capacitive sensor and an exemplary capacitor, which are supplied with voltage pulses generated by switches from a reference and output voltage, charge converter a voltage, a discrete integrator, and a square pulse generator.

A disadvantage of the known converter is the small range of converting capacitance to voltage, which is limited by the level of the output voltage corresponding to the initial value

sensor capacity, and the maximum allowable output voltage.

The purpose of the invention is to expand the conversion range.

The goal is achieved by providing partial compensation of the charge of the capacitive measuring probe. Structurally, this is implemented as follows. A capacitance-to-voltage measuring transducer containing a source of constant stable voltage, two switches, a measuring sensor and an exemplary capacitor, a charge-to-voltage converter, discrete

5 an integrator and a square pulse generator, a second constant voltage source is introduced, the polarity of which is opposite to the polarity of the first voltage source, the third switch and the second exemplary capacitor, the potential electrode of which is connected to the output of the third switch, the control input of which is connected with the output of the generator of rectangular pulses, the first information inputs of the first and third switches are connected, respectively, with the outputs of the first and second sources stable voltage, first information input

0 of the second switch is connected to the output of the discrete integrator, the second information input of the third switch is connected to the common bus.

FIG. Figure 1 shows the capacitance to capacitor to voltage transducer;

in fig. 2 - output characteristics of the converter in various operating modes.

The converter contains source 1

constant voltage, the second source 2 of a constant stable voltage, the polarity of which is opposite to that of the first, switches 3-5, measuring capacitive sensor b, first and second reference capacitors 7 and 8, converter 9 charging yes, voltage, discrete integrator 10 and generator of 11 rectangular pulses. The current electrodes of the capacitive sensor 6 and the first and second exemplary capacitors 7 and 8 are connected together and connected via the charge converter 9 to the discrete integrator 10. The potential electrodes of the measuring capacitive sensor 6 and the exemplary capacitors 7 and 8 are connected to the outputs of the switches 3-5. The first information inputs of switches 3 and 5 are connected respectively to the outputs of the first 1 and second 2 sources of constant stable voltage; the first information input of switch 4 is connected to

output discrete integrator. The second information inputs of the switches 3-5 are connected to the common bus of the device. The generator output of 11 rectangular pulses is connected to the control inputs of the switches 3-5 and the control input of the discrete integrator 10.

The Converter operates as follows.

The potential electrodes of the measuring sensor 6 and the reference capacitor 8 are fed by rectangular pulses of opposite polarity generated by the switch 3 from the voltage Uoi of the constant voltage source 1 and the switch 5 from the voltage Uo2 of the constant voltage source 2. The potential electrode of the reference capacitor 7 receives square-wave pulses generated by the switch 4 from the output voltage 1) The output of the measuring transducer. The switches are controlled by a generator of 11 rectangular pulses. The algebraic sum of the charges generated on the sensor and both of the exemplary capacitors is converted by the charge converter 9 to pulse voltages that accumulate in the discrete integrator 10. A change in the capacitance of the measuring sensor causes the charge to be unbalanced at the input of the converter 9 charge yes, which is compensated for by varying the output voltage of the discrete integrator 10.

The steady state operation of the device is characterized by equality

Uoi € 6 - Uo2Ce -See, (1)

where Do 1 and Uoa are the voltage of the first (positive) and second (negative) sources of a constant stable voltage, respectively;

Ce - sensor capacity;

C and Ce are the capacitance values of the first and second exemplary capacitors (the indices correspond to the indications of the capacitors in Fig. 1).

The sign of the output voltage of the device is opposite to the sign of the charge defined by the left side of equality (1).

The transfer function of the device is

, Uoi Ce-Uo2 Ce / Ol

willow .c)

In the particular case, picking up / Uoi / / UK /, you can write

and I | l. C6 C8 Uubix - Uoi Ј-.

The resulting expansion of the conversion range in the proposed device can be seen using the graph in FIG. 2, the output characteristics of which are given for the constant sensitivity of the converter (C const). Characteristic 1 in FIG. 2 corresponds to the transfer characteristic of the known device, described by Q as it is expressed by the expression “Uoi” (with

Uo2C8 0).

The capacity of the Ce sensor can be represented as

Ce Bon + ASB,

5 where RL is the initial capacitance of the sensor (for example, the capacity of an empty level sensor, the capacity of an empty or filled sensor with a dry medium, etc.):

0DSb - sensor capacity increment per

account change controlled parameter.

When a sensor is connected to the transducer with an initial capacitance of Bn at the output, its voltage will correspond to. the lower limit of the sensor capacitance. The upper limit of the sensor capacitance change will correspond to the maximum voltage Unbtx max.

0 at the output of the converter and for characteristic 1 its value will be Sq1. The measurement range, according to the capacitance, will be А А Се Скк1 - Снн (so, for example, in a known converter, the output voltage of the converter’s voltage varies from external - 2.25 to EXT max 9.0 V when the capacitance of the sensor changes from 5 pF Ck 20 pF, while the sensitivity of the converter will be S 0.45 V / pF.

0 Taking into account that the charge sign at the input of the charge converter 9 does not change during operation, the output voltage of the entire device will also not change its sign.

five ; When the second source 2 of a constant stable voltage and the second exemplary capacitor 8 is connected, partial compensation of the charges at the input of the converter 9 takes place, and the output voltage decreases accordingly. In the particular case, you can choose Uo2 or Ce in such a way that UoiCe io2Sz; then willows 0, and in order to reach the max sensor output,

5 value Sbc2 (feature 2 in Fig. 2). The measurement range in this case is A Sat Sbc2 - Csn, which indicates its expansion compared with the previous case.

The values of Uo2 and Ce can be chosen in such a way that the output voltage of the converter at the initial capacitance of the sensor is Ubn, is - Uw max. At the same time, the value of + iVy max reaches when the capacitance of the sensor is Sqbc (characteristic 3 in Fig. 2 |. And the range of variation is ACe Sbk.- Sbn (for example, while maintaining the former sensitivity of the S conversion) 0.45 8 / pF and changing the output voltage Uvykh from -9 V to +9 V, you can register a change in the capacitance of the sensor L S up to 40 pF, which exceeds the specified initial range of D Ce 15 pF by more than 2.5 times).

The converter can be adjusted at a predetermined or predetermined range of capacitance variation of the sensor. In this case, the voltage values of the source 2 of a constant stable voltage in the capacitance Cs. Of the second exemplary capacitor 8 are chosen from the condition Uoi (Сbr + 2 АСб Uo2Cs,

and then the capacitance value Su of the first exemplary capacitor 7 is selected at the value of the maximum in absolute value of the output voltage of the converter when the capacitance of the sensor is equal to Svn or Sbn + DSe.

Thus, the proposed capacitance-to-voltage converter allows to significantly expand the measuring range of the capacitance of the sensor. An additional advantage of the proposed converter is its ability to use it with sensors having a relatively large initial capacitance, which indirectly can provide an expansion of its field of application.

Claims (1)

Formula of measurement Measuring transducer with capacitive sensor, containing the first source of constant stable voltage, two switches, measuring capacitive sensor, first exemplary capacitor, charge-to-voltage converter, discrete integrator, square pulse generator, and the current electrodes of the measuring capacitive sensor and the first exemplary capacitor are connected and connected via the charge converter to the voltage to the input of the discrete integrator, the potential electrodes of the measuring capacitive sensor and the first exemplary capacitor are connected respectively to the outputs of the first and second switches, the second information inputs of the first and second switches are connected to the common bus of the converter, the output of the square pulse generator is connected to the control inputs of the first and second switches and the discrete integrator, the output of which is the output of the device, characterized in that, in order to extend the conversion range, In addition, a second constant voltage source is introduced, the polarity of which is opposite to the polarity of the first constant voltage source, a third switch and a second exemplary capacitor, the potential electrode of which is connected to the output of the third switch, the control input of which is rectangular pulses the first information inputs of the first and third switches are connected respectively to the outputs of the first and second sources of a constant stable voltage; the first information input the second switch is connected to the output of the discrete integrator; the second information input of the third switch is connected to the common bus of the device. four" A / g YA bfXMdto UA,  & btX.Mui & FIG. 2