Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1997-12-02
2001-02-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S678000
Reexamination Certificate
active
06194903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a circuit for acquisition of the capacitance or capacitance change of a capacitive circuit element or component, with a clock generator, a changeover contact controlled by the clock generator, a storage capacitor, a voltage source, and an evaluation stage in which one electrode of the capacitive circuit element or component is joined to the input of the changeover contact, the first output of the changeover contact is joined to the first electrode of the storage capacitor, the first electrode of the storage capacitor on the one hand is connected via a resistor network to the voltage source and on the other hand to the evaluation stage, and the second electrode of the storage capacitor is connected to a reference potential.
Within the framework of the invention, the “capacitance” is the capacitance value of a circuit element or component; a “capacitance change” is, consequently, the change of the capacitance value of a capacitive circuit element or component; the “acquisition” of capacitance or a capacitance change is both only a qualitative acquisition and also quantitative acquisition, therefore a genuine measurement; a “capacitive circuit element or component” is each circuit element and each component which has capacitive properties, and is often also called a capacitance, then the capacitance value not being intended; and a capacitive “circuit element or component” is especially a capacitor. However, within the framework of the invention, a “capacitive circuit element or component” is also the electrode of a proximity switch in cooperation with an influencing body, and for example, within the framework of the invention, a “capacitive circuit element or component” is also the capacitance which is represented by lines acting capacitively with one another. In the following, instead of “capacitive circuit element or component,” a sensor capacitor is always addressed without being associated with a limitation to a capacitor in a narrower sense.
Within the framework of the invention a “voltage source” is both a voltage source overall and also a terminal of this voltage source.
Finally, it is also noted that, by way of explanation, within the framework of the invention, “changeover contact” means a switch which is often also called a two-way contact and which, therefore, also has one input and two outputs, the input being joined either to the first output or to the second output.
The subject matter of the invention is not only a circuit for acquisition of the capacitance or capacitance change of a capacitive circuit element or component, but the subject matter of the invention is also a process for acquisition of the capacitance or capacitance change of a capacitive circuit element or component.
2. Description of Related Art
A circuit of the type described initially is known from published German Patent Application No. 40 39 006, but the circuit described therein is neither intended nor suited for use in a capacitive proximity switch. Rather, this circuit is used as a capacitance-frequency converter for generating an essentially rectangular converter output signal with a frequency that is dependent on the capacitance of a capacitor to be measured. For this reason, the circuit known from German Patent Application No. 40 39 006 delivers a frequency as the output signal as well.
In circuits of the initially described type, especially in the area of the sensor capacitor, a plurality of parasitic capacitances occur which can adulterate the measurement result. Basically, these parasitic capacitances can only affect the measurement result when a voltage excursion occurs and the amount of charge which has changed or been recorded in doing so has effects on the measurement result. If this voltage excursion is prevented, the parasitic capacitances inherent in the circuit can no longer have an effect.
SUMMARY OF THE INVENTION
In view of the foregoing, a primary object of the present invention is to devise a circuit of the initially mentioned type which is especially well suited for use in a capacitive proximity switch, but which can also be used advantageously for other purposes.
In addition, it is a further object to provide a circuit in which disturbance influences, such as component tolerance or temperature drift, can be balanced during the measurement.
Yet another object of the invention is to provide a circuit of the mentioned type with which parasitic capacitances that adversely affect the measurement result can be, for the most part or entirely, prevented.
The circuit according to a first teaching of the invention has an evaluation stage which is suitable for current evaluation, and with which, more or less, no voltage excursion occurs on the changeover contact. Within the framework of the invention current evaluation means, this means that essentially no voltage difference occurs at the outputs of the changeover contact. The changeover contact switches between the same voltages, so that no voltage excursion takes place at the input of the changeover contact.
If, as indicated above, no voltage excursion occurs at the input of the changeover contact, this means that the sensor capacitor is not sensitive and thus, for example, a proximity switch would not detect an object in the vicinity. For this reason, in the circuit of the invention, an auxiliary voltage is applied to the capacitive circuit element or component which is synchronized with the changeover frequency of the clock generator. Because a voltage excursion occurs due to the applied auxiliary voltage at the input of the changeover contact, and thus, also on the sensor capacitor, the capacitance or a capacitance change of the sensor capacitor can be measured.
According to one preferred, especially advantageous embodiment of the invention, the auxiliary voltage is temporarily turned off and a compensation measured value is generated from the measured value which then remains. If the auxiliary voltage is turned off, the sensor capacitor is electrically separated from the environment and only actions which arise within the circuit take effect. The measured value which then remains contains all offset errors of the entire circuit, for example, component tolerances, production tolerances, or temperature effects. The remaining measured value which results from all these interfering influences can now be stored as a new “zero value” and can be taken as a reference for further measurement. For this compensation cycle, besides turning off the auxiliary voltage, different techniques are possible which are all based on modulation of the auxiliary voltage in order to determine the actual capacitance value or the actual capacitance change of the sensor capacitor from the value which has been currently measured, all inevitable offset influences then being removed. For example, phase or amplitude modulation of the auxiliary voltage is possible, just like a phase reversal of the auxiliary voltage by 180° synchronously to the switching instant of the changeover contact.
The circuit according to the invention undergoes an additional improvement by the fact that there is a reference branch connected to the output stage for measuring a reference capacitance with a second changeover contact, a second storage capacitor and a second resistor network. Here, the reference capacitor, the second changeover contact, the second storage capacitor and the second resistance network are connected to one another in the same way as the sensor capacitor, the first changeover contact, the first storage capacitor, and the first resistor network are connected to one another. If the circuit according to the invention is made without such a reference branch, it can be built very easily, and due to the small number of parts, also very economically; but, it requires a relatively large useful signal, since difference formation is not possible. However, this circuit, especially in conjunction with the possible use of a compensation cycle, can be an economical alternative for a proximity switc
i f m Electronic GmbH
Metjahic Safet
Nguyen Vincent Q.
Nixon & Peabody LLP
Safran David S.
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