DD282515A5 - CIRCUIT ARRANGEMENT FOR MEASURING THE LEVEL OF NON-LEADING FLUIDS - Google Patents

Abstract

The invention relates to a circuit arrangement for measuring the filling level of non-conductive liquids. The sensor is used to measure the filling level of non-conductive liquids, in particular of fuel in vehicle tanks. The invention involves the use of a capacitive sensor, which acts in series with a fixed capacitor as a capacitive voltage divider, whereby means of Wechselspannungsmaesziger a filling level indirectly proportional Meszsignal is generated. Furthermore, the amplitude of the Meszsignals is generated by means of a capacitor / diode cleavage. With the aid of an inverting amplifier with a shifted transfer characteristic, the indirectly proportional signal is converted into a signal directly proportional to the filling level, which signal can be used to drive display devices. The sensor can be used to measure the fill levels of any non-conductive fluid. {Fill level; non-conductive liquids; capacitive sensor; Fixed capacitor; capacitive voltage divider; Meszsignal; inverting amplifier; Transfer characteristic; indirectly proportional signal; direct proportional signal; Anzeigegeraet}

Description

For this 2 pages drawings

Field of application of the invention

The invention relates to a circuit arrangement for the capacitive measurement of the level of non-conductive liquids, in particular of fuel in vehicle tanks.

Characteristic of the known state of the art

It is known to measure fill levels capacitively by the change in the dielectric constant is evaluated in a measuring capacitor due to the filling level of the medium to be measured. The evaluation can be carried out so that the capacitance is measured directly by means of bridge circuits or by the utilization of the charging time constant or by the evaluation of the quality change of a resonant circuit or by the evaluation of the change of the complex resistance. Furthermore, it is known that the measuring capacitor in series with a. Fixed capacitor switched and the voltage across the measuring capacitor is evaluated. In addition, it is known that this capacitive voltage divider is driven by an AC voltage and the Mbßspannung is evaluated and displayed via a voltage doubler. These known methods have the following disadvantages with regard to use in motor vehicles:

- The measuring voltage or the display is the fill level inversely proportional and is influenced by on-board voltage fluctuations.

- The transmission of the measured values is subjected to interference under the conditions of a motor vehicle.

- The realization of these methods is costly.

Object of the invention

The aim of the invention is to provide a measuring arrangement which makes it possible, under the particular conditions of use in vehicles and self-propelled agricultural machinery, to measure the level of fuel in tanks and display it directly in proportion.

Explanation of the essence of the invention

The object of the invention is to develop a circuit arrangement which transforms the level of non-conductive liquids into a directly proportional electrical signal.

According to the invention the object is achieved in that the sensor is designed as a capacitor whose field space is filled with the measuring medium, that changes its capacity depending on the filling level. This measuring capacitor is connected in series with a fixed capacitor, both of which act as a capacitive voltage divider on the generated by a generator square wave constant amplitude and frequency. The voltage across the measuring capacitor changes in proportion to the level indirectly proportional. With the help of a Diodon / capacitor circuit combination, the amplitude of this voltage is obtained, which is loaded high-impedance with a voltage follower and then fed to a display amplifier. The display amplifier must operate in an inverted manner so that the opposite tendency between level and measuring voltage is raised and a direct proportionality between the two quantities is established. On the output side, the power amplifier generates either a current or voltage signal, which is displayed with the help of a measuring instrument.

embodiment

The invention will be explained in more detail below using an exemplary embodiment. In the accompanying drawing show:

Fig. 1: measuring capacitor

Fig. 2: Block diagram of the sensor electronics

Fig.3: Circuit diagram of the sensor electronics

Fig.4: Transfer characteristic of the measuring amplifier.

A coaxial measuring capacitor 1 is located vertically in the liquid 2, whose level h is to be measured (FIG. 1). The advantage of the coaxial arrangement is in the fail-safe mode and in the damping effect on the liquid inside in case of vibration of the liquid container 3. The inner electrode 4 is connected to the sensor electronics, the outer electrode 5 to ground.

This measuring capacitor 1 in series is a fixed capacitor 6, wherein both are supplied with a square wave voltage, which generates the generator 7 (Fig. 2). The voltage U _m across the measuring capacitor is inversely proportional to the filling level h of the liquid side, since the following applies:

C _x (h) + C ₁

With the help of the peak value evaluation 8, the amplitude of the measuring voltage U _{m is} supplied to a display amplifier 9, which operates as an inverting amplifier. This ensures that between level h and output voltage U, the display amplifier, a proportional behavior exists. With the help of a measuring mechanism 10, this output voltage can be displayed.

The square-wave generator 7 is implemented in CMOS technology, the minimum variant being supplemented by R1 in order to achieve instantaneous oscillation (FIG. 3). After the rectification and the smoothing by means of the diode / capacitor circuit 11, the amplitude of the measuring voltage at the input of the voltage follower 12, which terminates this circuit part high impedance. This will not stress the voltage across C2. At the output of the voltage follower is a level-dependent DC voltage, which, however, is inversely proportional to the level h. The display amplifier 9 is realized as an inverting amplifier and thereby generates a directly proportional relationship between output voltage and level. However, in order to prevent modulation in the case of an "empty" measuring capacitor, ie at maximum measuring voltage U, _n , the transfer characteristic of the inverting amplifier must be shifted out of the coordinate origin ve by the value of the maximum input voltage U, _m , _{x of} the inverting amplifier (FIG This geschiel it with the help of the resistors R 2 and R 3. The output voltage U, can be converted, for example, for display instruments in motor vehicles in a current signal or directly displayed.

Claims (2)

1. Circuit arrangement for measuring the level of non-conductive liquids in motor vehicle tanks, wherein a capacitor arrangement changes its capacity by the liquid height and a direct proportionality between level and electrical output signal is produced, characterized in that this capacitor is connected to the Systemrnasse and is acted upon by a square wave voltage , is converted into a signal proportional to the level by means of any known peak detection circuit and an inverting amplifier having a shifted transfer characteristic. 2. A circuit arrangement for measuring the level according to claim 1, characterized in that said inverting amplifier has a shifted transfer characteristic, whereby the direct proportionality between level and electrical output signal is produced.