DE1616049B2 - CIRCUIT FOR CONVERTING A PHYSICAL MEASUREMENT VARIABLE SHOWN AS A CHANGE IN ELECTRICAL RESISTANCE INTO A CURRENT PROPORTIONAL IN THE REVERSE OF THIS - Google Patents

Description

The present invention relates to a circuit for converting a change in electrical resistance depicted physical measured variable, in particular the temperature, reversed into one of these proportional current.

A variety of circuits are known in which one as an electrical resistance change The physical measured variable shown is converted into one of these proportional currents. It will for example, made use of Wheatstone bridge circuits, in which instead of a source of known DC voltage or a constant one or two of the four common resistors illuminated photocell can occur (German patent specification 677 631). Furthermore, an in the circuit made use of switched-on amplifier, in which its output current as Image of the measured variable is exploited (Soviet patent specification 150 170).

In many applications, however, it is desirable to have the change in electrical resistance shown Convert the measured variable into one of these inversely proportional currents, so that a linear dependence is obtained. This often facilitates an evaluation and further processing of the measurement data. So far have been conversion tables are often required for such a conversion. Area of application for a direct Conversion to the inverse function occurs in many mathematical and computer systems. the Conversion into an inverse function is particularly desirable where the change in resistance physical measured variable shown in a non-linear function, such as B. a thermistor, whose resistance does not change linearly with temperature. If the acquisition of the measured variable with With the help of the zero method, the readjustment for the adjustment had to be carried out nonlinearly so far, which has a variable resistor special non-linear characteristic required.

The object of the invention is to provide a simple circuit for converting a change in electrical resistance mapped physical measured variable into one of these inversely proportional current create. This object is achieved according to the invention in that one of four resistance branches existing, from their output to their input diagonal via an amplifier in the sense of a Self-balancing feedback bridge circuit is used in which the resistance converter a source of constant direct voltage for the branch containing the measured variable in series with the resistance converter is switched on and in which the amplifier output current represents the converted measured variable.

The bridge circuit according to the invention can be a servo device for acquiring position data which is a function of the relationship between a reference voltage and a measured one Represent current, with these position data indicating a variable quantity to be measured, z. B. the variable resistance of a thermistor when these variable temperature conditions can be calibrated.

Further features of the invention and its advantages can be found in the following description in conjunction with the drawings. Show here

1 to 4 are circuit diagrams of bridge circuits used according to the known prior art,

F i g. Figure 5 is a circuit diagram of a bridge circuit embodying the basic principles of the present invention represents

F i g. 6 shows a modification of the bridge circuit according to FIG. 5, including a branch to maintain of the comparison and

F i g. 7 shows a circuit diagram of an exemplary embodiment of a bridge circuit according to the invention; including a servo device.

ίο As is well known, a bridge adjustment is to be retained or the zero state, a readjustment element is often used. The readjustment ' element can be used in the in FIG. 2 to 4 and described in more detail below be.

In Fig. 1 form the resistors R ₁ , R ₂ , R ₃ and R ₄ . one bridge branch each. In series with resistors R ₃ and R ₄ . is z. B. switched a sensor device, with z. B. a thermistor whose resistance is R _x . Let the line resistance of the thermistor be R _L ; if the resistance of the thermistor feed is high in relation to the resistance of the thermistor to be measured, a third resistor R _{L is} provided in order to obtain equal currents in the two halves of the bridge when the latter is balanced. The between the terminals 10 and 11 in F i g. 1 to 3 flowing current should now, as desired, be opposite to the change in resistance of the thermistor, for example. Furthermore, the readjustment element in FIG. 2 to 4 the starting points E ₁ and E ₂ are kept at zero for all changes in the resistance of the thermistor.

The bridge circuits in FIG. 2, 3 and 4 are similar to those in FIG. 1 except that in FIG. 2 the resistor R ₄ . through a variable resistor P ₄ . is replaced; in FIG. 3 the resistor R _{3 is} replaced by the resistor P ₃ , and in FIG. 4, the resistor R _{3 is} replaced by the variable resistors P ₁ and P ₂ .

In Fig. 2, the readjustment is carried out by a variable resistor P ₄ . reached in the measuring branch, "whose change in resistance is then complementary to the change in resistance of the thermistor.

In Fig. 3, the variable resistor P _{3 is} in the opposite position to that in FIG. 2 is arranged, whereby the change in resistance P ₃ then corresponds to the change in resistance of the thermistor.

In Fig. 4, two variable resistors P ₁ and P ₂ running in the same direction are used, where P ₁ keeps the current in both bridge halves the same, and the variable resistor P ₂ serves to balance the bridge and keep the output voltages E ₁ and E ₂ at zero. If, as in the case of a thermistor, the resistance to be measured is a non-linear function of a variable quantity (the resistance in a thermistor is a non-linear function of temperature), the readjustment must also be non-linear in order to produce a linear output voltage.

If the thermistor is now used to measure a temperature over a range of 50 ° F (= 10 ⁰ C) or more, the following difficulties arise with the circuits according to FIGS. 1 to 4:

The change in the resistance of a rheostat,

as shown in FIG. 2 and 3 is used, a contact noise is introduced directly into the readjustment.

Changes in the slope of the resistance-temperature curve are still directly as a gain change in the downstream of the output of the bridge Amplifier introduced. In addition, there is a non-linear variable resistor or one connected in parallel Rheostat required.

In Fig. 4, the potentiometers running in the same direction can generate a direct contact noise that affects the Current regulation of the bridge circuit can have a detrimental effect. Also is the gain control parallel potentiometer limited. In addition, from an economic point of view, the use of a pair of synchronous, non-linear potentiometers is undesirable.

According to the present invention, the aforementioned difficulties become thereby avoiding a fixed tension in one of the branches of a conventional bridge of the one shown Kind of like B. in Fig. 1, is introduced. By introducing such a fixed voltage, it is possible to determine the resistance of the thermistor by the current flowing in the bridge is determined without having to resort to the described readjustment elements. Of the The reason for this is clear from the following analysis.

The current state of solid state operational amplifiers leaves it out from an economic point of view appear desirable to the measured variable

equal to the function X = K · - ^ -, or at least

almost equal to this function, where K is a constant and R _{x is} the resistance whose value is to be determined. If a thermistor is used, a resistor R _s (not shown) can be connected in parallel to the thermistor so that the non-linearity deviates from the linear curve by only a few percent within the measuring range.

It has now been found that, by introducing a fixed voltage E _re , into a branch of a bridge, a measuring circuit is formed with which it is possible to obtain the reciprocal value of a resistance. This can be achieved in that the current flowing through the bridge is changed until an adjustment is achieved, the two halves of the bridge having the same current. Such a change in the current can be achieved by the in FIG. 6 and described later can be achieved. First, however, with reference to FIG. 5 performed the following:

It is assumed that the bridge output voltage E ₁ - E ₂ = 0, ie the bridge is balanced. If R ₁ = R ₂ , then the bridge current / in R ₁ and R ₂ must be the same so that £, - E ₂ = 0.

The following equations can then be set up:

described embodiment. By using a fixed reference voltage E _re , a balanced bridge with line compensation is achieved, with the variable current being used as the new adjustment variable. To use this bridge you must

1. the comparison is retained,

2. The current can be determined.

To maintain the adjustment, the variable E ₁ -E ₂ can be fed to an operational amplifier with a differential input (FIG. 6). The output current of the amplifier gives the supply current for the bridge. The input to the amplifier is connected in such a way that there is negative feedback (readjustment). If the gain of the amplifier is large enough, E ₁ - E _{2 will be} infinitesimally small and the bridge balance is thus maintained.
The following applies:

J =

£ 2,

ref

R _x

_β _ ^2E ref . Rl +

¹ - ρ τ

Ri + Rl

if A = gain:

₁ - L ₂ ) =

_ref

where E _{ref is} the reference voltage, R _{x is} the resistance of the thermistor, R _{1 is} the resistance value of one of the bridge branches and R _{L is} the resistance of the thermistor line. The variable e _t is the voltage at the input terminals of the bridge circuit.

A conventional potentiometric servo system can of course be used to measure the output current I and the span

voltage drop at one in the amplifier output circuit

switched on fixed resistor can be removed.

However, it is more advantageous to change the ratio of the

Output current as a function of E _{re /} to be obtained,

which can be done as follows:

Equation (1) was:

2E _re

E ₁ = E ₂
y IR _L = y IR, + i- IR _x - E _n ,

K ₁

If the drop in a bridge leg is compared to E _re , the ratio of the two can be determined. 1
With - ^IR 2

where r is a ratio and .R _{2 is} the resistance of a bridge branch,

y / R ₂ = f E _n,,

The current / is thus inversely proportional to R _x or the resistance of the thermistor in the one inserted in (2), results in:

_L - R ~

One sees that - ^ - what is called practically linear

Function of T (ambient temperature) is of interest, is directly proportional to r , and also independent of E _{re /} .

Equation (3) can be implemented very easily in terms of circuitry, as shown in FIG. 7, where the potentiometer P _{1 has} a ratio R for "£ _iusg JE _ref .

P ₁ is actuated by the servo motor M via the differential servo amplifier S until the adjustment is carried out, the position of the potentiometer P _{1 then being} an indicator for the ratio r , which in turn is a measure

for the required size - = - is.

^K x

The following then applies to the adjustment of the servo differential amplifier:

γ R ₂ = r £,

re />

20th

which corresponds to equation (3).

The circuit in FIG. 7 has the following characteristics:

1. The position of the potentiometer P ₁ is an indicator for the ratio r, which is a measure for

the size you are looking for is - ^ -.

2. The servo gain is just as linear as the function r.

3. E _ref is not critical and only affects the gain of the system.

4. The voltage at the unknown R _x is constant, which can be of interest where insulation losses of the line wires depend on the magnitude of the voltage or the polarity.

Claims (2)

Patent claims: 1. Circuit for converting a physical change represented as a change in electrical resistance Measured variable, in particular the temperature, into one of these inversely proportional currents, characterized by one consisting of four resistance branches, from its output on their input diagonal via an amplifier in the sense of a self-adjustment Bridge circuit in which the resistance transducer for the measured variable is contained Branch connected in series to the resistance converter a source of constant DC voltage and in which the amplifier output current represents the converted measured variable. 2. A circuit according to claim 1, characterized in that the amplifier output current is fed to one input terminal of a servo differential amplifier [S) _{, the other input terminal of which is connected to the wiper of a potentiometer (P 1} ) lying parallel to the source of constant DC voltage, and that the output voltage of the servo differential amplifier (S) via a servomotor (M) _{controls the wiper of the potentiometer (P 1} ) so that the position of the potentiometer wiper is a measure of the measured variable. 1 sheet of drawings