DD221847A1 - CAPACITIVE TRANSDUCER - Google Patents

Abstract

The invention relates to a capacitive transducer which enables the direct measurement of very small capacitances and which delivers an analogue electrical output signal as a function of the measured capacitance. The aim is to capture very small capacities directly with little effort on components. The task is to directly measure in the range of only a few picofarads with high accuracy and good temperature and long-term stability. This is achieved by the fact that the capacitance to be measured with an auxiliary capacitance, which form a capacitive voltage divider and an electronic switch in series and this voltage divider via the electronic switch to a stabilized voltage rechargeable, the capacity to be measured and the auxiliary capacity by one parallel connected electronic switch can be discharged and the partial voltage of the voltage divider via another electronic switch tapped and passed to an integration capacitor. The invention is suitable for all capacitive encoders as transducers, in particular for measuring the smallest capacitances. Fig. 1

Description

Capacitive transducer

Field of application of the invention

The invention relates to a capacitive transducer, which allows the direct measurement of very small capacitances and which supplies an analog electrical output signal as a function of the measured capacitance. Capacitive transducers find, for example, in the measurement of small paths and angles or in the detection of valve, pointer, diaphragm positions or levels, e.g. Oil, gasoline, water and the like., Especially in chemical plant, machine tool u. a. Application.

Characteristic of the known technical solutions

It is known from DD-WP I36 548 to measure an unknown capacitance C _x by continuously charging and discharging C _x in rapid alternation with a defined current and a defined frequency, wherein the charging current is sensed via the voltage drop across a resistor and is output as an output signal. Above all, these transducers have the disadvantage that they require a very stable generator which controls the transhipment transistors. Sin such generator can only be realized with a high circuit complexity. In addition, when measuring small capacitances, the generator frequency must be set very high in order to ensure sufficient

get signal '. However, the frequency increase in practice limits, so that capacity measurements of a few picofarads can not be made exactly.

From DB-AS 2 148 775 it is also known to measure the unknown capacitance C "in such a way that C is charged with a constant current from a DC source until the voltage at CL. Connects a connected Schmitt trigger and this drives the discharge transistor, the C discharges again. This process repeats itself at a frequency that is proportional to the size of the capacitance of C _Y. The output pulses of the Schmitt trigger are integrated and output as an analog signal.

Also, this converter is also not suitable for measuring small capacitances, since parallel to G, the interfering input capacitance of the Schmitt trigger and the frequency of the trigger is limited upwards.

Object of the invention

Bs is the object of the invention to provide a capacitive transducer which is able to directly detect very small capacitances with little effort on components.

Explanation of the essence of the invention

It is an object of the invention to form the capacitive transducer so that it can be measured directly with him in the range of only a few picofarads with high accuracy and good temperature and long-term stability. According to the invention this is achieved in that the capacitance to be measured with an auxiliary capacitance and an electronic switch is connected in series, wherein the capacitance to be measured and the auxiliary capacitance form a capacitive voltage divider and this voltage divider via

the electronic switch can be charged to a stabilized voltage, the capacitance to be measured and the auxiliary capacitance can be discharged by a respective parallel electronic switch and the partial voltage of the capacitive voltage divider is tapped via another electronic switch and directed to an integration capacitor.

Further features of the invention are that instead of the electronic switch, which picks up the partial voltage of the capacitive voltage divider, preferably a diode is used and the capacitive voltage divider is constructed as a differential capacitor. .

embodiment

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

Fig. 1 shows the circuit diagram of a capacitive transducer, Fig. 2 shows the partial circuit diagram of FIG. 1 with differential capacitor.

An unknown capacitance C to be measured is connected in series with an auxiliary capacitance Cjj and an electronic switch T 1. The capacitive voltage divider formed from C _x and Cg is connected via C _x to the reference potential and via T 1 to a stabilized voltage source U ₃ . In each case, an electronic switch T 3 »T 4 is located parallel to C _x and Cg. The connection point of C__ and Ctt is connected via a further electronic switch T 2 to an integration capacitor C and to the input of a differential amplifier 2. The second input of the differential amplifier 2 is at a Heferenzspannung U ^.

ι "

In operation, a generator 1 controls the electronic switches T 1; T 2 and T 3; T4 periodically with antiphase

to signals T, so that T 1 and T 2 in alternation with T 3 and T 4 are continuously opened and closed. In the phase in which T 1 and T 2 are conducting, the capacitive voltage divider C _x I Cg is charged to the voltage _{U stat)} , and via the electronic switch T 2 a charge equalization takes place on the integration capacitor C 1. In the phase in which T 3 and T 4 are conducting, the partial capacitances Cg and C _{x are} discharged again. The integrating capacitor C ₁ retains its charge since T 2 is now closed.

The current charge reversal of the capacitive voltage divider C} Ctt and the resulting charge flow over T 2 let the integration capacitor C ^ produce a voltage whose size depends on the size of the capacitance C ". This voltage compares the differential amplifier 2 with the reference voltage U _r ^ and forms the output voltage U ^.

As electronic switches transistors were used in the embodiment. According to the functional task of the electronic switch T 2, the installation of a diode is often sufficient.

When measuring 'small paths and angles with capacitive displacement and angle sensors they are often designed to achieve a linear measurement characteristic as a differential capacitor. In Fig. 2, an application example of the capacitive transducer is shown with such a differential capacitor. The differential capacitor 3 forms the capacitive voltage divider needed for the measurement. The function is analogous to the embodiment of Figure 1, wherein the electronic switch T 1 again provides for the charging of the voltage divider and by the electronic switch T 3; T 4 the partial capacities are discharged again and again.

The inventive capacitive transducer is simple in construction, allows the direct measurement of very small capacities with very good temperature and long-term stability and allows the increased use of easily manufactured capacitive Meßgrößenaufnehmer in practice.

Claims (3)

invention claim 1. Capacitive transducer? which enables the direct measurement of the smallest capacitances and which supplies an analogue electrical output signal as a function of the measured capacitance, characterized in that the capacitance (C ") to be measured has an auxiliary capacitance (Cg) and an electronic switch (T 1 ) is connected in series, wherein the capacitance (G _x ) and the auxiliary capacitance (Cg) form a capacitive voltage divider and this voltage divider via the electronic switch (T 1) to a stabilized voltage (U ₃ ^ ^) is chargeable, the measuring capacitance (O) and the auxiliary capacitance (Ctt) by a parallel electronic switch (T 3, T 4) are dischargeable and the partial voltage of the capacitive voltage divider via an electronic switch. (T 2) is tapped and passed to an integration capacitor (Cj). V, 2> capacitive transducer according to point 1, geke η η · i ei σ h η et ^: dadu rc h, - that instead of the electronic switch (T. 2) preferably a diode is used. 7 ^v . " 3 .. '. "Capacitive'Meßwahdler after point Λ and 2, ge ken η ζ ei ch η etd ad urch that the capacitive voltage divider is constructed as a differential capacitor (3). '- For this A page Drawings · -