RU108853U1 - COMPLEX RESISTANCE METER WITH COMPENSATION OF PARASITIC PARAMETERS - Google Patents

Abstract

 A complex resistance meter with spurious compensation, containing an AC reference voltage source, a reference resistor, three switches, an operational amplifier, a phase detector, a phase-shifting circuit and a voltmeter, characterized in that two inverting amplifiers with unit transmission coefficients, a correction capacitor and two parallel capacitors connected in parallel respectively to the measured complex resistance and a reference resistor, two-input summing an amplifier with adjustable transfer coefficients for each of the inputs, the first input of which is connected through the first inverting amplifier to the first terminal for connecting the measured impedance, to the second terminal of which the output of the summing amplifier is connected through a correction capacitor, while the second input of the summing amplifier is connected through the second inverting amplifier to the switching contact of the second switch.

Description

The utility model relates to electrical engineering and can be used to measure the components of capacitive and inductive objects.

Known devices for measuring the complex resistance [1], based on measuring the frequency of the signal of a high-frequency generator, in the frequency-setting circuit of which is included the measured element, a bridge measurement model with subsequent voltage measurement in the measuring diagonal of the bridge.

A device based on measuring the voltage at the midpoint of a voltage divider formed by the measured capacitance and resistance of the measuring shunt [2]. The disadvantages of these devices is the lack of compensation for spurious parameters.

A device [3] is known, which allows measuring the components of complex resistances, but with insufficient accuracy since it does not provide for compensation of spurious parameters during measurement.

The purpose of the utility model is to increase measurement accuracy.

Figure 1 shows the structural diagram of the proposed meter.

The meter contains an AC voltage reference source 1, first 2 and second 3 switches, an operational amplifier (op amp) 4, a phase detector 5, a third switch 6, a phase-shifting circuit 7, a voltmeter 8, the first 9 and second 10 inverting amplifiers with a unity gain, two-input summing amplifier 11 with adjustable transmission coefficients for each of the inputs, a correction capacitor 12, a model resistor 13 with a parallel parasitic capacitance C _n1 , the measured complex resistance 14 with parallel a flush-connected parasitic capacitance C _n2 , the first 15 and second 16 input terminals of the measuring circuit. The first input of the summing amplifier 11 is connected through the first inverting amplifier 9 to the first terminal 15 for connecting the measured impedance, to the second terminal 16 of which the output of the summing amplifier 11 is connected via a correction capacitor 12. The second input of the summing amplifier 11 is connected through a second inverting amplifier 10 to the switching contact second switch 3.

To measure capacitive objects, the first and second switches are set to position C, and for measuring inductive switches to position L.

In the case of measuring the components of a capacitive object, the device operates as follows.

Based on the operating conditions of the operational amplifier, covered by negative feedback (OS) voltage, we write the equation of currents

Where - current flowing along the OS circuit;

- current flowing in a direct circuit;

- the current flowing through the correction capacitor 12 (C _K ).

We write equation (1) as the sum of the currents flowing along all branches formed by the elements of the measuring circuit and spurious parameters,

Where - current flowing through the reference resistor;

- current flowing through a stray capacitance connected in parallel to a model resistor;

- current flowing through the measured complex resistance;

current flowing through a stray capacitance parallel to the measured complex resistance.

Substituting the values of currents into expression (2), we obtain

Where - the output voltage of the operational amplifier 4;

R ₀ is the resistance of the reference resistor;

ω is the frequency of the source 1 of the reference voltage ω = 2πf;

C _n1 C _n2 - spurious parameters;

C _k is the capacity of the correction capacitor;

Z _x is the measured complex resistance;

R ₁ , R ₂ , R ₃ - resistance of the resistors of the summing amplifier 11.

To compensate for spurious parameters, it is necessary to fulfill the conditions

Where - output voltage of source 1;

- output voltage of the operational amplifier 4.

From (3) and (4) we obtain the values of the transmission coefficients for the first and second inputs of the summing amplifier 11

Given the fulfillment of conditions (3) and (4), the voltage at the output of the operational amplifier 4 can be written

When measuring the imaginary component (7), the signal from the source 1 is supplied to the control input of the phase detector 5 through the phase-shifting circuit 7 and the switch 6, which is in position X for measuring the reactive component. The resulting voltage, proportional to the reactive component, is measured with a voltmeter 8.

When measuring the active component, the signal from the source 1 is supplied to the control input of the phase detector 5 through the switch 6, which is set to the position R for measuring the active component. The resulting voltage is also measured with a voltmeter 8.

For the case of measuring inductive objects represented by a sequential equivalent circuit, we obtain the current balance expression, substituting the values of the currents flowing along the corresponding branches in expression (2):

where L _X is the measured inductive object.

Substituting into the expression (8) the values of the coefficients (5) and (6), we obtain the voltage at the output of the operational amplifier 4 for the inductive object.

In this case, the measurement of the real and imaginary components of the voltage (9) occurs similarly to the measurement of the components for a capacitive object. Analysis of the operation of the meter taking into account spurious parameters is as follows.

We write the equation of currents.

+ = - -

Substitute the values of the currents in the branches for a capacitive object

From (10) we express the value of the output voltage,

Where

Similarly, we obtain the expression for the output voltage of amplifier 4 for an inductive object, taking into account spurious parameters

Where

A comparison of expressions (7) and (11), as well as (9) and (12) shows the increase in accuracy achieved in the proposed meter.

Thus, using a converter, by eliminating the influence of spurious parameters, it is possible to increase the accuracy of measuring the components of the complex resistance.

Bibliography

1. Lame B.P., Moiseev Yu.G. Electroradio measurements. M .: Radio and communications, 1985, p. 199, 203

2. Atamalyan E.G. Instruments and methods for measuring electrical quantities, M .: Higher school, 1989, p.302, Fig.13.18.

3. USSR copyright certificate No. 779916, cl. G01R 27/02, publ. 11/15/1980

Claims (1)

A complex resistance meter with spurious compensation, comprising an AC reference voltage source, a reference resistor, three switches, an operational amplifier, a phase detector, a phase-shifting circuit and a voltmeter, characterized in that two inverting amplifiers with unit transmission coefficients, a correction capacitor and two parallel capacitors connected in parallel respectively to the measured complex resistance and a reference resistor, two-input summing an amplifier with adjustable transfer coefficients for each of the inputs, the first input of which is connected through the first inverting amplifier to the first terminal for connecting the measured impedance, to the second terminal of which the output of the summing amplifier is connected through a correction capacitor, while the second input of the summing amplifier is connected through the second inverting amplifier to the switching contact of the second switch.