DE1591927B2 - A measuring bridge arrangement fed by direct or alternating current with a variable resistor as a measured value sensor - Google Patents

Description

35

The invention relates to a measuring bridge arrangement of the Wheatstone type for equal or Alternating current with a resistance that changes due to an influencing variable χ, its change linearly simulated by one or two variable resistance dividers in a freely selectable measuring stick will.

Usually the Wheatstone bridge is used to measure electrical quantities. In addition, can In connection with suitable transducers, however, they also have a broader field of application in the electrical field Find measurements of other physical quantities. - When using the bridge can a distinction can be made between two measurement methods. With one method, the so-called Deflection method, the bridge is adjusted for a certain amount of the measured variable (fixed point); the output voltage of the bridge is zero. If the measured variable changes, the bridge is detuned; the output voltage is different from zero and a measure of the measured variable itself the other method, the adjustment method, is the bridge with each measurement process with the help of a Adjusted actuator, the measured value being read from its position.

The two methods mentioned, which are used in various ways in measurement technology, have various advantages and disadvantages.

So is z. B. at a bridge to the deflection method, which includes a variable resistor with the process variable X R (X), the output voltage is not a linear function of X. The meter + aX)

(1.1)

If the bridge is adjusted for a point (e.g. zero point, X = O, Y = O), equation (1.1) changes over to

1 + aX = 1 +

Y,

(1.2)

Y.

If you now want a relationship between the two quantities Y and X in such a way that Y = X , care must be taken that the expression

—5 ^Σ - takes on the value 1, or - which is the same a K ₀ j.

is - the value a is to be simulated with sufficient accuracy using the ratio R _y : R _Oy. This requirement is all the more difficult to meet, the larger the range of variation of X and Y becomes with defined error limits. It is therefore usually necessary to correct the ratio R _y : R _Oy by adjustment. In general, R _y · Y is a variable step resistance, the division of which cannot easily be changed, so that a correction of the ratio R _y : R _Oy can only be achieved via R _Oy . This in turn changes the previously calibrated zero point position, so that the calibration then amounts to an iterative process that can sometimes be quite time-consuming (e.g. in the case of temperature measuring devices).

The invention is based on the object of creating a measuring arrangement which is simple and precise calibration allows without losing the advantageous properties.

The solution to this problem with a measuring bridge arrangement fed by direct or alternating current, whereby the output voltage of a fixedly balanced bridge, which is dependent on a measured variable (X) and which contains a measured value transducer based on a change in resistance in one branch, is replaced by the voltage which is dependent on a readjusting variable (Y) is simulated on a resistance divider, is characterized according to the invention in that the supply voltage U for the simulation resistance divider with the partial resistances or the partial conductance at a level determined by the bridge partial factor is derived from the bridge supply voltage by means of switching means, so that a linear relationship Y = Const · X exists.

The invention can be illustrated in the example according to FIG. 2 explain in more detail. The circuit includes a Wheatstone bridge with resistance dividers 1 and 2.

As is well known, the bridge transmission factor results from the difference between the transmission factors the divisors 1 and 2 too.

A _B (X) = - _μΐη Κμ = —L

^ + • Κοχ + ^ χ '- * K ₁₊ K ₂

(2.1)

Now the bridge is adjusted for a reference point (R _x -X = 0, R _y ■ Y = 0), ie it is

= μ- (2.2)

^30th

35

Let it be ζ. Β. μ = -γ provided, then R ₃ = R _0x . The relation (2.1) thus becomes

2R _{0x +} R _x -X 2 '

(2.3)

R - X
1 = 2 (2R _{0x +} R

40

45 This follows for the bridge output voltage

R _x -X

_Rx . _x)

u _s .

(2-4)

This voltage is to be simulated using a further divider 3 with the resistors R ₀₉ and R _y · Y , which is fed by the voltage U. Whose output voltage is U ₇ (Y)

R _v -Y

^ Oy

U (2.5)

Equations (2.4) and (2.5) become identical if the following conditions are met:

U =

Us

2
= nR _r

(2.6)

These conditions are easy to meet: - = r is

can be tapped off at the divider 1, η results from the selection of the resistance R _y and R _Oy can then be easily adjusted to the exact value by calibration at a further reference point. As you can see for yourself, the requirement X = Y is thus fulfilled. These considerations are of course only valid as long as the divider 3 can be regarded as having a high resistance to the parallel-connected resistor of the divider 1. It can be seen that, even if the dimensions are unfavorable, the target can be reached much faster during the calibration.

These problems do not arise in the case of a construction as an alternating current bridge, in which the divider 1 can be implemented by the tapped winding of a transformer. 3 shows such a circuit in which, as a modification of the simulation divider in conductance circuit with the conductance values G _Oy ; G _{y is} executed. In a further development of the inventive concept, the bridge can also be supplemented by a further divider 4 for measuring negative X (FIG. 4).

For this purpose 2 sheets of drawings

Claims (3)

Patent claims: 1. A measuring bridge arrangement, fed by direct or alternating current and intended to measure a physical quantity (X) , which contains a permanently balanced bridge from the dividers (1) and (2) with a measured value transducer based on a change in resistance and whose values depend on the measured quantity (X) dependent output voltage is simulated by the voltage dependent on a readjusting variable (Y) at a resistance divider (3), characterized in that the supply voltage U for the simulation resistance divider (3) with the partial resistances (R _Oy ; R _y ) or the partial conductance values (G _Oy ; G _y ) is derived from the bridge supply voltage (U _s ) at a level determined by the bridge division factor (μ) by means of switching means, so that in the case of adjustment there is a linear relationship Y = Const · X. 2. Arrangement according to claim 1, characterized in that the variable resistor (R _y · Y) or control conductance (G _y · Y) of the replica resistor divider (3) used for display has a linear division. 3. Arrangement according to claim 1 and 2, characterized in that to correct a non-linearity of the transducer at taps of the variable resistor (R _y · Y) or conductance values (Gy · Y) of the replica divider (3) correction resistors are connected in a known manner. must then be provided with a non-linear scale division of X. If the voltage in the bridge diagonal is measured without power, this disadvantage can be avoided by switching to the known half-bridge or full-bridge circuit with two or four oppositely changing resistors. Assuming an extremely constant supply voltage, these disadvantages are offset by the advantage of a particularly simple calibration of the measuring device: the zero point and sensitivity can be adjusted independently of one another. Sometimes it is difficult or not possible to use the transducer with the required accuracy to be built up with two exactly opposing resistance characteristics (e.g. resistance thermometer). If one does not want to forego the absolute linearity of the display, one is forced to use the adjustment method to fall back on, the advantages of which are known, but the disadvantage of which becomes clear when the transducer shows only small changes in resistance and requires extremely high accuracy will. The calibration of the bridge then causes difficulties in practice, which is a simple example shows (Fig. 1). Let the transducer with its resistance R (X) dependent on the measured value χ be given by the relationship R (X) - R _ax (l + aX). Let R ₀₁ and jR ₀₂ be fixed resistors, and let R _y Y be a . Changeable resistance at whose position Y the measured value X is to be read. The compensation condition of the bridge is then: