SU1718126A1 - Ac transformer bridge - Google Patents

Abstract

The invention relates to a measurement technique and can be used in instruments for measuring parameters of complex conductivities, as well as non-electrical quantities, using impedance sensors, in particular for measuring the specific electrical conductivity of electrolyte solutions by four contact electrolytes.

Description

The invention relates to a measuring technique and can be used to measure the parameters of complex conductances, as well as non-electrical values using impedance sensors, in particular, when measuring the specific electrical conductivity of electrolyte solutions by contact four-electrode cells.

AC transformer bridges are known for measuring small impedance parameters.

The disadvantage of these devices is low measurement accuracy in the presence of significant shunt conductivities in cases where the resistances of the supply wires are a significant amount. In addition, shunt conductivities, having a reactive nature, also sharply limit the frequency range of known devices.

An alternating current transformer bridge is known, which contains a generator, a voltage transformer with a primary winding connected to a generator and two secondary windings, a first current terminal connected to the input of the first voltage follower and, through an exemplary measure, to the beginning of the first secondary windings, second current terminal connected to the common point of the device, potential terminals connected via an equilibrium detector and directly to the beginning and end of the second secondary winding.

A disadvantage of the known device is the low measurement accuracy due to the strong influence of leaks with windings on other elements, as well as low measurement accuracy.

The closest in technical essence to the present invention is an alternating current transfer bridge, comprising a generator, a voltage transformer, an exemplary measure, a measurement object, two voltage repeater and an equilibrium detector. To eliminate the influence of parasitic conductivities, individual

windings are made with shielded wire.

The bridge contains a voltage generator, a voltage transformer with a primary

winding and three secondary windings, model measure, current clamps and potential clamps for connecting a measurement object, for a voltage repeater (PN), an object for measuring an equilibrium detector (DF).

Famous bridge works as follows. The voltage of the secondary winding of a voltage transformer is applied through an exemplary measure to current clamps.

and causes a current flowing through the measurement object. A voltage follower is used to form at the end of the winding a voltage equal to the signal on the current clamp. In this case, the series-connected winding and the sample measure operate in the short-circuit mode, and the current x flowing through the measurement object is equal to

Um4

l

Z0

0

five

0

five

where Um4 is the voltage across the winding;

2.o - resistance exemplary measure.

The voltage drop across the measurement object created by the indicated current is removed using potential terminals and fed to successively connected secondary windings and an equilibrium detector. The latter compares the voltages supplied to it and changes the number of turns of the winding so that, at the time of equilibrium, the voltage on the winding is equal to the voltage on the measurement object. The equilibrium condition is

h gp4 1

Gx - -,

YY15 about

where Gx is the conductivity of the measurement object;

m, trig- the number of turns of the secondary windings.

The voltage follower and the secondary winding together with the repeater create the necessary voltage at the end of the secondary winding and at the same time serve as sources of protective potentials to which the shields of the wires connected to the potential are connected.

A disadvantage of the known bridge is the inaccurate formation of a given current mode, due to the following error components: the first is the load error caused by connecting the secondary winding of the first transformer to low-resistance supply circuits, resulting in a voltage drop U2 on the ohmic resistance of the winding, which leads to error 5i:

TSN-TSF1)

U u

5r

wherein URX + UR + UR + U2;

URX - voltage drop between potential leads:

UR, Us - voltage drop between the corresponding potential and current outputs:

U URX + UR + UR, (2)

U URX + U R + U R is the voltage UH with no current flowing through the winding.

For a more detailed consideration of the error, consider the equivalent circuit of the PN1 power supply circuit. In alternating current, the capacitor of the filter Sf and the sources of direct current power are short-circuited. Then from (1) and (2) for this scheme we have

L -U2-g2. URX + UR ° t - 11 -

U, + Z2 Provided

UV UR URx

URX + UR + UR

WHAT RF GI

, then d «-yy-; Ty

Rf

The personal resistance value of the filter of Russia is 100 Ohms, and for the winding, for example, 10 turns of 0.01 Ohms. Then d 0.02%. . The second component of the error is due to the non-ideal parameters of the operational amplifier PN 1, in particular, the final value of the coefficient of attenuation of the common-mode signal CSF. This error can be estimated by the formula

, 1, H + l2 + UR T

02 K (I1 + UR XF

For an operational amplifier with an acceptable unit amplification rate and an output slew rate, the typical CSF value is 80 dB. Therefore & 0.02%.

For a known bridge, there is another source of error due to the non-ideal suppression of the common-mode component in the differential detector.

five

0

five

0

five

0 5

0

five

equilibrium torus. This error is commensurate with the error 62.

Hence, the total measurement error for the known bridge is 6 0.04-0.05%.

The indicated error components reduce the measurement accuracy.

The purpose of the invention is to improve the measurement accuracy. t:.

The goal is achieved in that an alternating current transformer bridge containing a generator, an equilibrium detector, a first voltage transformer with a primary winding connected to a generator, and with the first, second and third secondary windings, the first and second voltage followers, and the first the voltage follower is connected through a standard measure of resistance to the beginning of the adjustable first secondary winding, and the output to the end of the same winding, two current and two potential terminals for connecting the measuring object and, the first current terminal is connected to the common busbar, and the first potential terminal is connected to the input of the second voltage follower, while the connecting wire is shielded, the screen of which is connected to the first output of the second voltage follower, the second output of which and one of the generator terminals are connected with a common bus, a second voltage transformer is introduced to the first and second windings / a second current terminal for connecting the measurement object connected via the second secondary winding of the first voltage transformer to the input terminal The first voltage follower, the second potential terminal through the third secondary winding of the first voltage transformer connected in series, the secondary winding of the second voltage transformer and the equilibrium detector are connected to the common bus, the beginning of the primary winding of the second voltage transformer is connected to the first output of the second voltage follower, end its and the second output of the first repeater voltage are connected to the common bus. The second and third secondary windings of the first voltage transformer and the secondary winding of the second voltage transformer, as well as the connecting wires of these windings, are made with shielded wire, the shield of the wire connecting one of the secondary winding leads to the input of the first voltage repeater stress and eq

The secondary winding wound of the second transformer is connected to the common bus.

The positive effect is achieved due to the fact that the voltage follower transmits a relatively small (relative to the balancing signal) signal of the relative common bus.

The drawing shows the scheme of the proposed bridge.

The parasitic bridge contains a generator 1, a transformer 2 voltage with a primary winding 3, the beginning of a secondary winding 4, which through an exemplary resistance measure 7, a voltage follower 8 (PN) is connected to the end of this winding, a measurement object 9, the first current output 10, of which through the secondary winding 5 is connected to the input of the PN 8, the first potential output 11 through the third secondary winding b and the secondary winding of the transformer 12 is connected to the equilibrium detector 13, the second potential output 14 via the voltage follower 15 second winding of the transformer 12. The outputs of both repeaters voltage detector equilibrium and second current output 16 connected to a common bus.

The proposed bridge works in the following way.

Under the action of the applied voltage in the circuit of the measurement loop 9, a current flows. The CL 8 serves to form at the connection point of the winding 5 to the exemplary measure 7 of the potential resistance close to zero, with the result that the measurement object is in the mode of a predetermined current. The winding 5 has the same number of turns with the winding 6 and serves to compensate for the voltage drop between the potential clamps of the object. As a result, the voltage transmitted by the PN 8 is significantly reduced, since

Ro R +, where Po is an exemplary measure of resistance;

R M R is the resistance between the potential and current leads, and the requirements on the accuracy of the transfer coefficient and the input resistance of the voltage follower are significantly reduced. Thus, between the pins 10 and 16 of the object current flows

0

five

five

leads to a fall of the stress D D l Q (

five

0

five

This current

nor on the elements R1, Rx and R of the object to be measured. The voltage drop between potential terminals 11 and 14 is removed with the aid of PN15 and transformer 12 and compared with the voltage across winding 6 with an equilibrium detector. The bridge is balanced by changing the number of turns of the winding 4. At the time of equilibrium, the voltage on the winding 6 is equal to the voltage drop between terminals 11 and 14. It follows from the equilibrium condition that the measured conductivity Gx is determined as follows. general expression

Gx Ux Rx me Ro

where me is the number of turns of the third secondary winding of the transformer 2 voltage. At the moment of equilibrium, the current in the compensation circuit (pin 11, winding 6 and the secondary transformer 12 DS) does not flow, hence the RI and Ra parameters do not affect the measurement result.

To eliminate the influence of parasitic conductivities on the result of the comparison, winding 6, the secondary winding of transformer 12 and the connection between them are performed with a shielded wire, the screen of which is connected to the common bus on the input DR side. The winding 5 is also performed with a shielded wire, and the screen is connected to the output of the PN 8. Thus, equipotential protection of the transmitted signals is provided.

In the proposed bridge, the output (power circuits) of the PN 8 is connected to the common bus. As a result, the loading error 3i, characteristic of the known bridge, is absent, and the error due to the final pressure of the in-phase component & is defined as

52 1 / XF,

where Ksf is the ratio of the common-mode signal of the operational amplifier.

For typical operational amplifiers, the KSF is 80 dB. therefore

(52 0.01%.

The advantage of the proposed bridge is a higher measurement accuracy, compared to the known one, it increases by 4-5 times.

0

, If4 .Wo

I - U.

ts ro

where ts is the number of turns of the primary winding of the transformer 2;

d is the number of turns of the first secondary winding of the transformer;

Uo-voltage generator.

Claims (2)

Claim 1. An alternating current transformer bridge comprising a generator, an equilibrium detector, a first voltage transformer with a primary winding connected to a generator, and with the first, second and third secondary windings, the first and the second the second voltage follower, the input of the first voltage follower is connected through an exemplary measure of resistance to the beginning of the adjustable first secondary winding, and the output to the end of the same winding, two current and two potential terminals for connecting the measurement object, the first current terminal connected to the common bus and the first potential clamp is connected to the input of the second voltage follower, the connecting wire is shielded and the screen of which is connected to the first output of the second voltage repeater, the second output which and one of the outputs of the generator are connected to a common busbar, characterized in that, in order to improve measurement accuracy, a second voltage transformer is inserted into the primary and secondary windings, a second current terminal for connecting the measurement object is connected through the second secondary winding of the first transformer voltage with the input of the first voltage follower, the second potential terminal through the third secondary winding of the first voltage transformer connected in series, the secondary winding of the second transformer voltage and equilibrium detector connected to a common bus, the beginning of the primary winding of the second voltage transformer is connected to the first output of the second voltage follower, and the end with a common bus. 2. The device according to claim 1, wherein the second and third secondary windings of the first voltage transformer and the secondary winding of the second voltage transformer, as well as the connecting wires of these windings are made with shielded wire, the screen of the wire connecting one of the terminals of the secondary winding to the input of the first voltage follower is connected to the output of this voltage follower, and the secondary winding screen of the second transformer is connected to the common bus.