SU1532885A1 - Capacitance to frequency converter - Google Patents

Abstract

The invention relates to a measurement technique and can be used in meters with capacitive sensors. The goal is to improve measurement accuracy. The device contains reference voltage source 1, switches 3 and 4, comparator 6, integrator 7, measured and exemplary capacitors 8 and 9, unbalance amplifier 10 of the signal, logic unit 11, synchronous detector 12. Introduction to a triangular voltage generator 5 device , logic unit 11 and analog inverter 2 allowed to reduce conversion errors by balancing the instantaneous values of the currents flowing through the reference and measured capacitors. 2 Il.

Description

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tin

Shigz

The invention relates to a measuring technique, in particular, to devices for measuring electrical circuit parameters.

The aim of the invention is to improve the conversion accuracy by balancing the instantaneous values of the currents flowing through the reference and measured capacitors. ten

Fig. I shows the structural scheme of the device; 2 shows timing diagrams for his work.

The device contains a source of 1 reference voltage, an analog inverter 15, 2 electronic switches 3 and 4, a triangular voltage generator 5, a comparator 6, an integrator 7, measured 8 and exemplary 9 capacitors, an unequal amplifier 10, a logical unit II, synchronous detector 12.

The outputs of the source 1 are connected to the inputs of the generator 5 and through the terminal 3 the first input of the compressor 6, the first terminals of the capacitors 8 and 9 are connected to each other and to the input of the amplifier 10, the outputs of which are connected to the signal inputs of the syn- chronous detector 12, the output of which 0 is connected to the first input of the switch 4 and, through an analog inverter 2 with its second input, the output of the switch 4 is connected to the input of the integrator 7, the output of which is connected to the second input of the comparator 6 and the second output of the capacitor 9, the second output of the capacitor 8 It is connected to the analog output of the generator 5, the control output of which is connected to the first input of the logic unit 11, the second input of which is connected to the output of the comparator 6 and the control inputs of the switches 3 and 4, the first and second outputs of the logic unit 11 are connected 5 respectively with the first and second control inputs of the synchronous detector 12.

35

Logical -block 11 contains logs

AND 13, OR-NOT I4, OR, 15,16 and triggers 17,18, and. the first input of the element AND 13 is connected to the first input of the element OR-NOT 14, with the input of the trigger 1-7 and is the first, the first input of the logical block I1, the second input of the element AND 13 is connected to the second input of the element OR-NOT 3 4 and is the second input of the logical

50

5 0 5

five

0

block 1, the output of the element AND 13 is connected to the first input of the element OR 15 and is the second output of the logical block 11, the output of the element OR NOT 14 is connected to the second inputs of the elements OR 15 and 16, the output of the element OR 16 is the first output of the logical block 11, the output of the OR element 15 is connected to the inputs R of the flip-flops 17 and 18, the output of the trigger 17 is connected to the input T of the flip-flop 18, the output of which is connected to the first input of the OR 16 element.

Synchronous detector 12 contains capacitors 19, 20, switches 21, 22, capacitor 23, amplifier 24, the control inputs of switches 21 and 22 being respectively the first and second control inputs of synchronous detector 12, the first outputs of switches 21 and 22 are connected between with the inverting input of the amplifier 24 and the first output of the capacitor 23, the second output of which is connected to the output of the amplifier 24 and is the output of the synchronous detector 12, the inputs of the switches 21 and 22 are connected to the inputs of the synchronous detector 12 via condensation tori 20 and 19.

The device works as follows.

Generator 5 On its information output, feeds a triangular waveform with a stable frequency and amplitude equal to ± Uon, where f Zol is the output voltage of source 1 (see Fig. 2.1). Analog Inverter 2, switches 3 and 4, comparator 6 and integrator 7 are a voltage-to-frequency converter (VFD). At the output of the integrator 7, a triangular voltage is formed (see Fig. 2.2) with amplitude fUon, whose span is stably maintained within this value by the comparator 6. As the voltage at the output of the integrator 7 rises to + Uon, the comparator 6 is triggered, at first the input of which is supplied + Uon, through switch 3 from source 1, and to the second input - output signal of integrator 7. The output signal of comparator 6 controls the operation of switch 3 and 4, therefore, the voltage at the first input of comparator 6 becomes -Uon, so how warm pb first input of the comparator 6 via the switch 3 is connected to the output

five

-Uon of the source 1. Simultaneously with the switching of the switch 3, the switch 4 switches, so that the sign of the voltage applied to the input of the integrator 7 -U0x changes to the opposite + Ugx 31, the count of the use of the analog inverter 2. The output signal of the integrator 7 changes the direction of integration and when a voltage of -1TOP is reached at its output, the comparator 6 is triggered again, the switches 3 and 4 occupy the initial positions, and the whole process repeats periodically. The higher level of the output voltage from the synchronous detector 1 is fed to the input of the VFR, i.e. to the input of the analog inverter 2 and the normally closed input circuit of the switch 4, the higher the frequency of the output tre-. the carbon signal at the output of the integrator 7. The output frequency of the triangular signal at the output of the integrator 7 and the rectangular signal at the output of comparator 6 is determined by the expression

 2G Ua t. "Ta1--

on and

0

5 0 5

Who is the signal at the output of the integrator 7,

dUon ---- - the rate of increase of the linear portion of the triangle output

signal on

generator 5.

Logic unit 11 and synchronous detector 12 operate in such a way that the output voltage of the latter is proportional to the difference in currents in capacitors 8 and 9 at those times when the currents in them flow in the opposite direction. The operation of these blocks is explained in the diagrams in Fig. 2: Fig. 2.1 shows the voltage at the analog output of the generator 5; Fig.2.2 is the voltage at the output of the integrator 7; Fig.2.3 - logical signals at the control output of the generator 5; Fig.2.4 - logical signals at the output of the comparator 6; Fig.2.5 - logical signals at the first output of the block 11; Fig.2.6 - logical signals at the second output of the block 11; Figure 2.7 is the voltage at the output of the synchronous detector 12. When the currents flowing through the capacitors 8 and 9 at time points

f.

L

-output frequency converter capacitance-frequency

-time integration of integrator 7, obtained

s

where U & x is the input voltage of the integrator 7; 2)) - output voltage of the integrator

-and,

7, changing

+ U,

from

oo d ° tiop h ЈM - the integrator time constant 7.

The magnitudes of the currents flowing through the exemplary 9 and the measured 8 capacitors are determined by the angle of inclination of the triangular signal at the outputs of the generator 5 and the integrator 7.

P Vx /

dIJcn dtT

(2)

 Cv ex

dUon

"S .." in

at.

1and „n

(3)

the rate of increase of the linear section of the triangle

thirty

g35

40

t

pa45

- h

.- t

4 f etc.

neither

 t

equalized, the voltage at the output of the amplifier 10 at these times will be zero. When an equilibrium condition is reached at the specified time intervals, the currents flowing through the capacitors 8 and 9, defined by expressions (2) and (3), are equal to

ai ™

dtl

g ai00 0 at

one

WITH.

(four)

you can make you

From the expression (4)

water that is a transformation function

this converter capacity in

the frequency will be determined by

by:

WITH

WITH

(five)

where fx is the output frequency of the transducer;

ffl is the generator frequency 5. The advantage of the invention in comparison with the prototype is a reduction in error when working with differential capacitive sensors, since in this case the instantaneous values of the currents flowing through the measured and reference capacitors are balanced, as well as

expanding functional capabilities-U, thanks to the ability of the new converter to measure the capacitance value at C0 Cx; C 0 Cx; C 0 "Cx; however, the frequency of the output signal may be less, equal to, or greater than the frequency of the generator 5.

 Formula of invention

Capacitance-to-frequency converter containing a source of reference voltages, first and second electronic switches, exemplary and measurable capacitors, - the first terminals of which are connected to the input of a non-equilibrium signal amplifier, a synchronous detector, whose analog inputs are connected to the output of a signal amplifier imbalance, integrator and comparator connected in series, characterized in that, in order to improve accuracy,

four %

-%

Jij fL- ij is pi, a igiq n s

,

tt ts% q

Compiled by O. Bogdanovich Editor O. Spesivykh Tekhred L. Oliynyk Proofreader E. Lonchakova

Order 8095/51

Circulation 714

VNIIPI State Committee for Inventions and Discoveries at the State Committee on Science and Technology of the USSR 113035, Moscow, Zh-35, Raushsk nab. 4/5

0

It has an analog inverter logic unit and a triangular voltage generator, the outputs of the reference voltage source are connected to the inputs of the first switch and the triangular voltage generator, the analog output of which is connected to the second output of the measured capacitor, and the control output is connected to the first input of the logic unit the second input of which is connected to the output of the comparator and the control inputs of the switches, the output of the first switch is connected to the second input of the comparator, the input of the integrator is connected to the output th second switch, the first and second outputs of the logic unit connected to the first and second synchronous detector by control inputs, whose output is connected to a first input of the second comparator and the analog inverter - a second input of the second comparator.

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Claims (1)

A capacitance-to-frequency converter containing a reference voltage source, first and second electronic switches, exemplary and measured j; capacitors, the first conclusions of which are introduced by an analog inverter ^ a logic block and a triangular voltage generator, and the outputs of the reference voltage source are connected to the inputs of the first switch and a triangular voltage generator, the analog output of which is connected to the second output of the measured capacitor, and the control output is connected to the first input of the logic unit, the second input is cerned connected to the output of the comparator and the control inputs of the switches, the first switch output is connected to the second input of the comparator, the input of the integrator soediryh connected to the input of amplifier imbalance signal, a synchronous detector, the analog inputs are. ' 'connected to the output of the nonequilibrium signal amplifier, an integrator and a comparator connected in series, characterized in that with the output of the second switch, the first and second outputs of the logic unit are connected to the first and second 20 control inputs of a synchronous detector, the output of which is connected to the first input of the second comparator and through an analog inverter to the second one, which, in order to increase accuracy, is in the input of the second comparator.