SU1762263A1 - Method of conversing capacitance of differential transducer - Google Patents

Abstract

Usage: in measurement technology, in particular in converters of capacitance into voltage or current. Summary of the Invention; periodic recharging of 2 capacitors and separation of TOKOS from both directions of recharging, proportional to the difference of currents of 2 capacitors. A particular feature of the invention is the additional isolation of currents from the second direction of the recharge, which allows for improved h / v interference immunity. 2 i l i.

Description

This invention relates to a measurement technique, in particular, to converters for capacitive sensors. It can be used to measure displacements, pressure, and other physical quantities.

The known method is based on periodically recharging the sensor capacitance and generating a result in the form of a voltage difference proportional to the sensor capacitance.

The disadvantage of the known method is low sensitivity, since only the discharge of the capacitance of the sensor is used to form the result. Another disadvantage of the method is low resistance to low-frequency interference.

There is also known a method based on the periodic recharging of the sensor capacitance and the formation of the conversion result in the discharge process.

The disadvantages of this method are also low sensitivity and low noise immunity.

The closest to the invention in its technical nature is a capacitance conversion method based on periodic recharging from a source of alternating voltage of the first and second capacitors, during which the recharge currents of capacitors of one direction are extracted, from which the output signal is proportional to the difference of the first and second signals.

A device that implements a known method, contains the first transistor with the first diode and the second transistor With the second diode. The emitters of the transistors are connected respectively to the first and second capacitors of the differential sensor, the common lining of which is grounded, the bases of the transistors are combined and connected ..:. output of a variable periodic voltage source. Transistors periodically switch from closed state to linear mode and back when the polarity of the alternating voltage changes. The currents in the collectors of the transistors depend on the capacitors of the capacitors Cxi and Cx2 and

(L

xj

GO

Yu

ON

WITH

 ; (Eyappl

their average (for the period of the signal of the source of alternating voltage) values are determined by the expressions

1 1cr K Cxi + 1psr1 Ik2cp C-CX2 + 1psr2,

where K is a coefficient proportional to the frequency and amplitude of the signal of the alternating voltage source; Psr1 and Psr2 are the average for the period of the signal of the alternating voltage source, the values of the components of the collector currents of the first and second transistors, due to low-frequency pickups on the capacitors of the sensor.

The collectors of the transistors are connected to the inputs of a differential amplifier, which produces an output signal proportional to the differential capacitance of the sensor, for example, current. described by expression

Out K (Cx1 - CX2) + (1psr1 - 1psr2) K (CX1 CX2) ± D1psr

OI Cxi - CX2, Alncp (1psr1 - 1psr2) O, BUT

For all-Cx1 / Cx2 D psrЈ 0, since Incpi and Psr2 are proportional to the values of the capacitances Cxi and Cx2. Thus, the known method does not ensure the stability of the conversion to low-frequency interference.

A disadvantage of the known method is also low sensitivity to changes in sensor capacitance, since the conversion result is determined by a signal proportional to the value of only one of the two oppositely directed currents that recharge each capacitor. The second current flows through the discharge circuit and is not used directly to obtain information about the capacitance of the sensor.

A second disadvantage of the method is the low noise immunity of the conversion due to the influence of low-frequency, in particular, network noise induced on the capacitors of the sensor.

The aim of the invention is to increase the sensitivity and noise immunity of the conversion,

This goal is achieved by the fact that in a known method of converting a capacitance of a differential sensor based on a periodic recharge from a source of alternating voltage of the first and second capacitors, during which the recharge currents of one of the single engravings are extracted, from which the output signal is generated.

a portion of the difference between the first and second signals, additionally allocates overcharge currents of a different direction, and the first and second signals form proportional differences of the overcharge currents, respectively, of the first and second capacitors.

Figure 1 shows the functional diagram of a variant of the device for implementing

10 ways; On fmg.2 - an example of the implementation of a differential amplifier, which is part of the device.

A device that implements the proposed method, contains the first and second

15 transistors 1 and 2 of the same type of conductivity, the bases of which are combined and connected to the output of a grounded source of alternating voltage 3, the emitters of the first and second transistors 1 and 2 are connected

20, respectively, with the plates of the first and second capacitors 4 and 5 of the differential sensor 6, the common plate of which is grounded, the collectors of the first and second transistors 1 and 2 are connected 25 with the first and second inputs 7 and 8 of the difference amplifier 9, the output of which is output 10 of the device , the third and fourth transistors 11 and 12, the type of conductivity which is opposite to the type of conductivity

30 of the first and second transistors 1 and 2, the bases of the third and fourth transistors 11 and 12 being combined and connected to the output of the alternating voltage source 3, the emitter of the third transistor 11 is connected to the emitter of the first transistor 1, the emitter of the fourth transistor 12 is connected to the emitter of the second transistor 2, the collectors of the third and fourth transistors 11 and 12 are connected respectively to the third 40 to it and the fourth inputs 13 and 14 of the difference amplifier 9.

Difference amplifier 9 (Fig. 2) contains resistors 15, .. 24, capacitors 25 ... 28, operational amplifier 29, the output of which is output 10 of the device. When implementing a differential amplifier circuit, it is necessary to fulfill the condition R15 R16 R17 R18, the preferred form of the output

50 of the source voltage 3 signal is sinusoidal.

The implementation of the proposed method is carried out as follows.

AC source 3

55 alternating periodic voltages are applied to the bases of all four transistors 1, 2, 11 and 12, in pairs, either in open or in closed state. With a positive half-wave of sinusoidal voltage, the first and second

transistors 1 and 2, and their emitter currents provide simultaneous recharge of the first and second capacitors 4 and 5 of the differential sensor 6 with currents of the same direction.

The collector currents of the first and second transistors 1 and 2 are the input currents of the first and second inputs 7 and 8 of the difference amplifier 9. The third and fourth transistors 11 and 12 are closed at a positive half-wave. With a negative half wave, they open, and the capacitors 4 and 5 of the differential sensor 6 are recharged by currents of a different direction. The collector currents of the third and fourth transistors 11 and 12 are the input currents of the third and fourth inputs 13 and 14 of the difference amplifier 9, respectively. The average (for the alternating voltage period) values of the collector currents of the first and third transistors 1 and 11 Ikicp and 1n2sr are proportional to the capacitance Cxi of the first the capacitor 4, and the average values of the collector currents of the second and fourth transistors 2 and 12 Ik2cp and Iki2cp are proportional to the capacitance Cx of the second capacitor 5 of the differential sensor 6.

The action of low-frequency (for example, with the frequency of the network) interferes with the appearance in the collector currents of all four transistors of the components + Psr1 ± 1 psr11 ± psr and ± 1PCg2 ± psr12 t G psr. Equality data is recorded taking into account the fact that the frequency of the signal of the source 3 of alternating voltage b2 10..4 SE4R is greater than the frequency of interference (the frequency of interference 50 Hz, and the optimum frequency of alternating voltage 1-2 MHz), i.e. the disturbance current can be considered constant during the alternating voltage period.

We assume that during the period of variable voltage, Cxi const and Cx2 const. The values of the components I ncpi and G psr depend on the values of the capacitances Cxi and Cx2 and for all Cx Cx2 there is a Gpcr G psr. On resistors 15 ... 18 and capacitors 25 ... 28 of the difference amplifier 9, constant voltages are selected that are proportional to the collector currents of the transistors. For ease of implementation, the resistors 15 ... 18 are chosen to be the same: R17 R18 R. These voltages are summed with the corresponding signs of the amplifier based on resistors 19 ... 24 and operational amplifier 2S. The output voltage of the device 10 is described by the expression:

VSbix10 {(fk1cp + I ncp) - (lk11cp + l ncp)

- 0k2cp + Gpsr) - (Ik12cp + G psr)} x

x R-Ay (Iklcp + Ik11cp) - (Ik2cp +

+ lk12cp) -R Ay 2 () x

xR-Ay 2K (Cxi-Cx2), (4)

where Ay is the gain transfer factor of the amplifier based on the operational amplifier 29 and resistors 19 ... 24; ikicp ikiicp icxi

And Ik2cp Ik12cp 1CX2.

Comparison of expressions (3) and (4}) allows us to conclude that the proposed method provides twice as much sensitivity and increased noise immunity of the conversion as compared to the prototype.

Claims (1)

Invention Formula A method for converting the capacitance of a differential sensor based on periodic recharging from an alternating voltage source of the first and second capacitors, during which the recharge currents of capacitors of one direction are extracted, from which they form an output signal proportional to the difference of the first and second signals, differing in. that, in order to increase sensitivity and noise immunity, the overcharge currents of the capacitors of another direction are additionally isolated, and the first and second signals are proportional to the differences of the overcharge currents of the first and second capacitors, respectively. s / 37  / J  7 J Ix -ts P% mnar "g (3 -t./7 74 /four Z / X7 J