DE3634245A1 - Evaluating electronics for differential resistances to be used in sensors - Google Patents

Abstract

As already described in Patent P 3524530.1, the measurement value in the evaluating electronics for differential resistances with a pulse-width-modulated squarewave output voltage, in which a capacitor is alternately charged via in each case one of the two part-resistances of the pick-up up to a reference voltage and then is recharged up to a second reference voltage, is, after low-pass filtering, strictly proportional to the measurement quantity. Supplementary to Patent P 3524530.1, further illustrative embodiments are presented here which are distinguished by a simple construction.

Description

The invention relates to evaluation electronics for differential resistors with a pulse-width modulated square-wave output voltage, in which a capacitor is alternately charged to the positive + U _ref or negative reference voltage - U _ref via one of the 2 partial resistors.

As already described in patent P 35 24 530.1, this shows Reloading procedure for the evaluation of differential resistance transducers due to the small number of components, low drift and the proportionality of the output voltage to the measurand is essential advantages over other circuits.

In one used in the prior art circuit (Fig. 1) _{2 are} simultaneously supplied to the voltage drop across the partial resistors R ₁ and R ₂ voltages U ₁ and U an addition and Subtraktionswerk. A measured value proportional to the measured variable is present at the output of the subsequent divider. In the measured value processing described, a series of mathematical links are carried out electronically, in which the stability of the components, especially the divider, are strongly temperature and time-dependent.

The task of processing measured values for a differential Resistance is solved according to the invention in that a con capacitor alternately over one of the two parts stands is reloaded.

The advantages achieved with the invention are in particular in the fact that with the help of this simple evaluation electronics Measured value proportional to the measured variable after low-pass filtering is obtained from the pulse-width modulated square-wave signal, the measured value of the stability of the capacitor to be charged is independent.

Some additional exemplary embodiments of the invention are shown in FIGS. 2 to 4.

In Fig. 2, a circuit is shown in which the capacitor Kon 8 via the switches 7 and 12 (similar to an "H-scarf device") is reloaded. In the circuit shown, the charging current flows through the resistor 1 and the switch 7 and thus charges the capacitor 8 . The counter electrode of the capacitor is connected to reference point A (here ground) via switch 12 . After the reference voltage + U _ref he was enough, the switches 7 and 12 are flipped so that point B now has the same potential as point A (here ground). This shifts the voltage on both electrodes of the capacitor 8 by the amount + U _ref , so that the voltage - U _ref is present at point C. The switchover process charges the capacitor at point C from - U _ref to + U _ref with the charging current via resistor 2 . The comparators can be connected so that either with a comparator and a switch, the capacitor voltages of points B and C are alternately compared with the comparison voltage + U _ref , or that at each of points B and C a comparator compares the voltages.

In Fig. 3 a circuit is shown in which the voltage divider consisting of the resistors 3 and 4 is connected directly to the output of the comparator. Here, a switching hysteresis is generated by coupling the comparator via the voltage divider. The output voltage of the comparator remains as long as the capacitor 8 has not yet reached the positive or negative reference voltage, at a constant level (control voltage).

If the comparator with symmetrical supply voltage operated, so the pulse width modulated rectangle take the output signal directly at the output of the comparator, the amplitude using reference voltage diodes can be stabilized.  

In Fig. 4 a circuit is shown in which the charging voltage U ₁ or - U _{1 is} tapped directly at the output of the comparator 9 , the current either by means of a switch or, as can be seen in Fig. 4, by 2 diodes the partial resistors 1 and 2 is branched.

Claims (4)

1. Evaluation electronics for a differential resistor with mutual recharging process (via RC element) and pulse-width modulated square-wave output voltage, characterized in that with differential resistors with a common voltage supply, the mutual recharging process, by switching the current direction on the capacitor (H circuit) by means of two switches is reached ( Fig. 1). Furthermore, only a comparison voltage U _{ref is} necessary here, since the reference potential (ground) on the capacitor is switched synchronously with the charging process. 2. Evaluation electronics according to claim 1, characterized in that the voltage divider for generating the reference voltage U _ref , is directly connected to the output of the comparator (comparator with hysteresis), the amplitude of the pulse-width modulated rectangular output voltage being stabilized by means of reference voltage diodes. 3. Evaluation electronics according to claim 1 and 2, characterized in that the charging voltage U ₁ or - U _{1 is} taken off directly at the output of the comparator.