HRP920220A2 - APPARATUS FOR WATER RESISTANCE COMPENSATION AND COMPLEX BRIDGING - Google Patents

Abstract

The line resistance compensation device and complex semi-bridge balancing is designed to eliminate the effects of line resistance in electrical measurements by a bridge. In addition, the device enables complex balancing of the measuring bridge. Reactive equilibrium is achieved by adjusting the potentiometer, which avoids the use of variable capacitors. This reduces the dimensions of the measuring device and increases the robustness. The new solution of the line resistance compensation device in the metering bridge is effective also in all cases where classical methods such as 3-wire and "Kreutzer" joint are not usable. The device is primarily intended for measurement in 1/4 and 1/2 bridge configuration, but using two such devices can easily be measured in full bridge configuration.

Description

Technical field to which the invention belongs:

The invention belongs to the field of electronic measuring technology. According to MPK, it belongs to group G01R 17/16.

Technical problem

In certain applications, the branches of the measuring bridge are physically distant and potentially at different temperatures. The physical distance brings into the equivalent scheme of the bridge the resistance of the lines, which leads to a measurement error. The resistance of the lines leads to a decrease in the sensitivity of the bridge, and different temperature coefficients of the resistance of the lines (copper) and the active element (manganine, etc.) or different temperatures of the bridge branches spoil the balance of the bridge and lead to a shift of zero (drift).

Balancing of the bridge is usually done with appropriate elements located in the measuring device. Some types of bridges or the design of the measuring device require alternating excitation of the bridge. With such a measurement, it is necessary to achieve a complex balance of the bridge. Balancing elements are potentiometers (for working balance) and variable capacitors (for reactive balance). Variable capacitors are relatively large and mechanically sensitive. A special problem arises when multiplexing many bridges to one measuring device. Then the overall dimensions of the balancing elements significantly exceed the size of the measuring device itself.

State of the art

In technical practice, two compensation connections are used to cancel the influence of line resistance. These are the 3-wire connection and the so-called "Kreutzer compound" described in patent file DBP 2314-754- (Federal Republic of Germany). Both compounds have advantages and disadvantages as follows:

3-wire connection

a) Advantages

The connection is very simple and requires only one additional conductor. Hence the name of the compound. It does not require any active electronic circuits. It can be added to the existing measuring device.

b) Disadvantages

The connection is efficient for 1/4 - bridge configuration. It requires equal resistances of the supply and drain lines to the active quarter of the bridge. Line resistance compensation applies to a balanced bridge. The joint does not ensure bridge balancing (Additional elements required).

"Kreutzer" compound

a) Advantages

The connection compensates for the influence of the lines in 1/4 and 1/2 - bridge configuration. It tolerates different resistances of the lines in each branch of the bridge. The power supply of the bridge elements is 4-wire.

b) Disadvantages

The joint requires a symmetrical inactive half of the bridge. The resistances of the supply and discharge lines to each element of the half-bridge must be equal. The connection is based on compensatory action on the source of bridge excitation, so it is not possible to add it to an already existing measuring device. Balancing of the bridge is not provided by this joint (Additional balancing elements required).

Description of the solution to the technical problem

The compensating circuit for canceling the influence of line resistance, shown in Figure 1, is basically designed for a 1/2 - bridge connection with a simple connection for a 1/4 - bridge connection. Switching is performed by replacing the impedance of one active branch (ZA or ZB) with a suitable stable resistor, and by short-circuiting D+ and S+, and A and A', that is, the corresponding terminals for the other branch.

The half-bridge of the nominal impedances ZA and ZB is fed by the excitation source E. The local feedback connection of the amplifiers A+ and A- compensates the voltage drop on the resistances r+ and r-. Amplifier A1 monitors the potential difference between points A and B (UAB) and controls the inverse current mirror. The inverse current mirror consists of a sink of current IA, and a source of current IB increased (or reduced) by current ΔI controlled by amplifier A1. The current difference equalizes the voltage drops on the resistances r1 and r2, that is, it equalizes the potentials of points A and B. This eliminates the voltage drop on the resistances r1 and r2 from the voltage distribution along the half-bridge. The compensation error is determined by the offset voltage of the amplifier A1, and the offset currents of the current mirror. By fine-tuning the current mirroring, the voltage UA changes, which is equivalent to changing the ratio of impedances ZA and ZB. By changing the ratio of currents IB and IA by module and argument, the bridge can be balanced in a complex manner.

The subsequent amplifier AG precisely defines the shield potential of the connecting lines of the half-bridge and significantly reduces the capacitive load of points A and B. The remaining capacitance is compensated by adjusting the reactive current mirroring component.

The inverse current mirror is shown in Figure 2. When analyzing the circuit, it was assumed that the input resistance of amplifiers A1, A2 and A3 is much higher than the other resistances in the circuit, so it can be ignored. It is also assumed that the offset currents create offset voltages on the resistances of the circuit far smaller than the offset voltages of the amplifier. After the solution of the system of nodal equations follows the expression for the current IB

IB= A IA+B(IAr1 + IBr2) + B e1 - C e2- D e3 1

that is:

IB= A IA+B (IAr1 + IBr2)+ Ios ; Ios=Be1-Ce2-De3 2

where factors A, B, C and D are:

[image] 3

[image] 4

[image] 5

[image] 6

The independence of IB on the resistances of lines r1 and r2 is achieved by setting the factor B to zero:

[image] 7

The resistance value of R1 is independent of condition 7, so adjusting R1 can be used to cancel the working component of the bridge unbalance. By analyzing the voltage distribution along the half-bridge, it follows for the voltage UA with adjusted line resistance compensation (B=O):

[image] 8

Expression 8 shows the dependence of the voltage UA on the factor A, ie: on the value of R1, that is, the voltage of the diagonal of the bridge is directly dependent on the choice of R1. Thus, R1 becomes an element for adjusting the working component of the balance of the bridge.

Reactive balance of the bridge is achieved by modifying the inverse current mirror assembly according to Figure 3. Potentiometer R8 is connected between hubs B and C, and its slider is connected to ground via capacitor C. Current drain I' and I" depends on the position of the slider of potentiometer R8:

[image] 9

[image] 10

which results in a change in the bridge diagonal voltage UΔ

[image] 11

where is:

[image] 12

the equivalent admittance entered in the bridge scheme depends on the position of the slider of potentiometer R8, and ZC and ZD are the impedances of the other half of the bridge. By setting Y to a value

[image] 13

the complex balance of the bridge is achieved by successively adjusting R1 (factor A) and the ratio R6/R7 (admittance Y).

Citation of the best way for commercial use:

In addition to respecting the general rules of electronic technology, it is necessary to ensure the following for optimal use:

1. To minimize drift and measurement error, operational amplifiers A1, A2 and A3 must be of high quality with offset voltages less than 100 μV and currents less than 10 nA.

2. Resistors R1, R2, R3, R4 and R5 must be elements of a hybrid resistance network with a good matching of the temperature coefficients of individual resistors.

3. The nominal value of resistors R1, R2, R3, R4 and R5 should be 100 Ohms. This ensures optimal operation of the 5 V bridge excitation device and 350 Ohm bridge branch resistance. (practical industry standard for measuring with strain gauges)

4. To adjust the working balance of the bridge, a stable resistor of 1 kOhm should be added in parallel to resistor R1, and a precision potentiometer of 20 Ohm in series.

5. Potentiometer R8 for adjusting the reactive balance should be of nominal value 1 kOhm and of high stability.

6. Capacitor C should be of nominal value 10 nF and of high stability.

7. The maximum output current of A1, A2 and A3 must be at least 20 mA for the nominal values of resistors R1 to R5 of 100 Ohms and the nominal resistance of the bridge branches of 350 Ohms, and the bridge excitation of 5 V.

Claims (3)

1. Device for compensating line resistance and complex balancing of an electric bridge according to Figure 1, indicated by the fact that one polysource of the bridge excitation E is connected to the non-inverting input of the operational amplifier A+ whose output through the conductor resistance r+ supplies one end of the impedance of the bridge branch ZA while the same end is ZA is connected to the inverting input of the amplifier A+ through the resistance of the second conductor r'+, and the other end of the impedance Z, through the resistance of the conductor r'+ is connected to the inverting input of the operational amplifier A+, and through the resistance of the conductor r'+ is connected to the current sink of the controlled inverse current mirror, while the source of the current mirror is connected through the resistance of the conductor r1 to one end of the impedance of the branch of the bridge ZB, which is connected through the resistance of the conductor r2 to the non-inverting input of the amplifier A1 and the operational amplifier AG whose output is connected to the inverting input of the amplifier AG and to all cable shields to the impedances ZA and ZB, and the output of the amplifier A1 is connected to the control terminal of the controlled current mirror, while the other end of the impedance ZB through the resistance of the conductor r_ connected to the output of the operational amplifier A_, and through the resistance of the conductor r'_ it is connected to the inverting input of the amplifier A_ whose non-inverting input is connected to the other pole of the excitation source of the bridge E. 2. Device for compensating line resistance and complex balancing of an electric bridge according to claim 1, with an inverse current mirror according to Figure 2, indicated by the fact that one end of the impedance of the bridge branch ZA is connected to the inverting input of the operational amplifier A1 through the resistance of the conductor r'1, and through the resistance of the conductor r1 connected to the non-inverting input of the operational amplifier A3, and one end of the resistor R1, the other end of which is connected to the output of the amplifier A1, and one end of the resistor R2, the other end of which is connected to the inverting input of the operational amplifier A2 and one end of the resistor R3 , while the other end of the resistor R3 is connected to the output of the amplifier A2 and one end of the resistor R4, the other end of which is connected to the non-inverting input of the amplifier A2 and one end of the resistor R5, and is connected through the resistance of the conductor r2 to one end of the impedance of the branch of the bridge ZB, and that end ZB is connected to the non-inverting input of the amplifier A1 through the resistance of the conductor r'2, while the other end of the resistance R5 is connected to the inverting input and from by amplifier A. 3. Device for compensation of line resistance and complex balancing of an electric bridge according to claim 1, with adjustment of the reactive balance of the bridge according to Figure 3, indicated by the fact that the output of the operational amplifier A is connected to one end of the resistor R1, and one end of the resistor R2, the other of which is end connected to one end of the potentiometer R8, the inverting input of the operational amplifier A2 and one end of the resistor R3, the other end of which is connected to the output of the amplifier A2 and one end of the resistor R4 while the other end of the resistor R4 is connected to the non-inverting input of the amplifier A2, one end resistor R5, and with the other end of the potentiometer R8 whose slider is connected to one end of the capacitor C while the other end of the capacitor is connected to the reference potential (ground).