Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2001-04-02
2003-07-08
Oda, Christine (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S662000, C324S680000, C324S686000, C324S690000
Reexamination Certificate
active
06590401
ABSTRACT:
TECHNICAL FIELD
The invention relates to a capacitive proximity switch for the evaluation of capacitance changes with an electrical alternating current measurement bridge of four bridge branches, which reach every two of four switching knots of the bridge, wherein an alternating current is positioned as a bridge feed voltage near two opposite, not neighboring switching knots, and respectively the two bridge branches between the two feed in switching knots build each of the two halves of the bridge, and there is an evaluated bridge diagonal voltage on the two remaining switching knots over diagonal voltage path, wherein the bridge diagonal voltage is removable, for detecting an approaching or moving away face, in particular an electrically bad conductive face or metallic face, which face is part of variable capacitor in one of the bridge branches of the bridge circuit, wherein at least one capacitor (capacitive bridge) or at least one capacitor and/or one resistor (capacitive ohmic bridge) is disposed in the remaining bridge branches as further reactences, wherein rectifiers are arranged to the rectification of the both bridge branch voltages separately according to respective bridge halves in the diagonal voltage paths and the diagonal bridge voltage is evaluated only after the rectification of the both bridge branch voltages as changing direct current corresponding to the capacitance change of the variable capacitor, according to the preamble of the claim one as well as method according to preamble of the claim 9.
STATE OF THE ART
The evaluation of small up to very small changes of capacitance is an always returning task in sensor technology. Very small changes in capacitance, namely in an order of magnitude of less than 10 fF, have to be reliably evaluated in particular in connection with capacitive proximity switches, wherein here in particular the stability against interferences as well as the temperature stability of the respective circuit assumes a central importance.
It is known to employ an oscillator for determining small changes in capacitance, wherein the oscillating amplitude of oscillator changes depending on the capacitance of the sensor. The size of the oscillation amplitude thus is a measure for the value of the capacitance of the sensor. Such a circuit is associated with the disadvantage that the circuit is not stable relative to temperature based on a principle, and therefore an eventually difficult to dimension temperature compensation is necessary in connection with such a circuit in most cases. Furthermore the quality and efficiency of the oscillator is low, wherefrom a broad band circuit with a bad electromagnetic compatibility behavior results.
Furthermore, it is known for the determination of small changes in capacitance to employ an oscillator, wherein the frequency of the oscillator changes depending on the capacity of the sensor. Again the disadvantage is associated with the employment of such an oscillator, that such a circuit is not stable relative to temperature based on a principle, and therefore here again an eventually difficult to dimension temperature compensation is necessary; also such circuit has a relatively bad electromagnetic compatibility behavior. so-called switched capacitor technique is furthermore known for determining small changes in capacitance, wherein a critical timing of the clock cycle signals is of disadvantage and wherefore an extremely stable clock cycle signal is required, which imposes an expensive switching technology depending on the method.
Also integrated circuits are furthermore known for the determination of small changes in capacitance. The integrated circuits exhibit the disadvantage that in most cases a ground free capacitance is necessary. In addition such circuits require in general a digital evaluation unit (mostly a counter) which means a large expenditure for switching technology. Such concepts are employed predominantly in the micro systems technology for these reasons.
It is furthermore known that the determination of small changes of an electrical value can be realized advantageously with the bridge circuits. The value to be measured is here compared with reference values, wherein these reference values are generated by similarly operating elements. Thus temperature influences can be effectively suppressed as long as the two bridge branches of the bridge have at all times the same temperature. Changes of the respective value are then presented as changes of the bridge diagonal voltage. The employment of reactances as bridge elements causes that the bridge has to be operated with alternating current. Thus also the bridge diagonal voltage represents an alternating voltage. The evaluation of the bridge diagonal voltage presents here frequently a problem according to the state of the art, because
the amplitude of the alternating voltage is very small based on the small change in capacitance, namely a few millivolts mV;
the frequency of the alternating voltage, which is employed for operating the bridge, and therewith also the frequency of the bridge diagonal voltage is located in the MHz region in order to avoid that the branch currents do not assume too small values given the small capacity values;
the bridge diagonal voltage in many cases in addition to the alternating voltage component also includes a common mode direct current component with a substantial larger as compared to the alternating current component.
Such previously known solutions are contained for example in the printed documents DE-C2-3143114, DE-A1-3911009, DE-A1-19536198, DE-A1-19701899, CH-558534, as well as in the EP-A1-0723166.
A circuit arrangement for eliminating the influence of a phase shift between the voltage potentials of the two measurement points of an alternating current measurement bridge with complex resistances is furnished, wherein the difference of the voltage potentials forms the measurement signal. A rectifier valve is disposed between the measurement points and the input enclosures of a circuit for forming the difference in each voltage path, wherein a storage disposed between the output of the rectifier valve and a reference potential common to the alternating current measurement bridge is connected following to each of the rectifier valves.
Capacitors employed as capacitive measurement value receivers within an alternating current measurement bridge, either in a bridge circuit with four capacitances or with two capacitances and two resistors, wherein in each case a capacitance is variable, are known from the literature location Heinz Schneider, Kondensatoren als Messwertaufnehmer (Capacitors as measurement value receivers), Elektronik-Applikation Nr. 14, Jul. 9, 1985.
A proximity switch with an alternating current measurement bridge is known by FR-A-2371676, which is subjected to an alternating current. The bridge branch voltages are rectified and subject to single electrical manipulations. Only after the rectification of the bridge branch voltages these as changing direct current as capacitive size are evaluated.
TECHNICAL PURPOSE
It is an object of the present invention to furnish a capacitive proximity switch as well as a method, which is highly sensitive to the approach of the object on one of its sides and by this allow a reliable evaluation of small changes in capacitance, wherein the circuit is to show a high resistance to interferences as well as a high-temperature stability based on its principle. Furthermore, proximity switch is to be realizable with comparatively small expenditure and therefore at low costs.
DISCLOSURE OF THE INVENTION AND ITS ADVANTAGES
The proximity switch according to the present invention comprises a flat multilayer circuit board comprising at least two electrically insulating layers, wherein an electrically conductingly intermediate player is disposed between the two electrically insulating layers as a first place of capacitance in one of the two bridge branches of one of the bridge halves and wherein a flat electrically conducting covering is placed on one of the two layers furnis
Kasper Horst M.
Kerveros James C.
Oda Christine
Pepperl & Fuchs
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