Circuit arrangement for evaluating an output signal and sensor device

Abstract

A circuit arrangement (10) for evaluating the output signal of an inductive or capacitive proximity switch (105) for switching operations in the low to medium frequency range with a transistor (T) in emitter configuration, at the base of which an input signal (Vdem) can be supplied, at the collector of which an output signal (Vs) characterizing a switching operation can be tapped and whose emitter is set to a predetermined potential (VRef), is characterized in that a coupling capacitor (Ck) is connected between the emitter and the base, which transmits disturbing short-term voltage spikes at the base directly to the emitter.

Description

[0001] The present invention relates to a circuit arrangement for evaluating the output signal of an inductive or capacitive proximity switch for switching operations in the low to medium frequency range. The circuit arrangement comprises a common-emitter transistor, whose base receives an input signal, whose collector receives an output signal characterizing a switching operation, and whose emitter is set to a predetermined potential. The relevant frequency range is defined by the IEC 609547-5-2 standard. Section 9.5.2 of this standard is particularly relevant for the proximity switches in question.

[0002] The invention further relates to a sensor device comprising an inductive or capacitive proximity switch and an evaluation circuit which comprises an oscillator, a demodulator for outputting the demodulated signal and a comparator for comparing the demodulated signal with a reference signal and for outputting an output signal depending on the comparison, wherein the comparator is realized by an aforementioned circuit arrangement. State of the art

[0003] Various circuit arrangements, for example a generic common emitter circuit, can be found in the book by U. Tietze and Ch. Schenk: Halbleiterschaltungstechnik, 10th edition, 1993 Springer, Berlin, ISBN 978-3-662-07640-8.

[0004] DE 102 14 201 A1 discloses a circuit arrangement for level adjustment, in which a transistor with two connected resistors and preferably a capacitor is provided, which is connected on the control side to a first supply potential and on the output side to a second supply potential terminal. The described level adjustment circuit has a high level of interference immunity and is suitable for transmitting signals up to 6.5 megahertz in frequency.

[0005] DE 11 2015 004 612 T5 relates to a capacitive detection device, which can also be combined with a heating element, particularly for use in vehicles, which can be manufactured cost-effectively and with low-frequency noise suppression.

[0006] DE 197 01 899 A1 describes a circuit arrangement for detecting the capacitance or a change in capacitance of a capacitive circuit 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 an electrode of the capacitive circuit or component is connected to the input of the changeover contact.

[0007] Proximity switches or proximity sensors are used in a wide variety of industrial sectors. For example, inductive proximity switches are used to detect the presence of ferrous metal or steel. These inductive proximity switches generate a high-frequency magnetic field on their face using an internal resonant circuit, also referred to as an oscillator. A metallic element located near the face is slightly magnetized, absorbing energy from the high-frequency magnetic field and thus reducing the oscillator voltage. A downstream comparator detects this and, at a set switching distance or oscillation level, controls an output switching amplifier, for example. This triggers a switching operation when the metallic object approaches the inductive proximity switch.Such proximity switches are used for contactless detection of approaching objects.

[0008] In a similar way, capacitive proximity switches can also be used for such purposes.

[0009] The switching distances can be adjusted by appropriate dimensioning of the oscillator and the evaluation unit.

[0010] Common-emitter bipolar transistors are particularly used as evaluation circuits for the output signal of an inductive or capacitive proximity switch for switching operations in the low to medium frequency range. An input signal can be applied to their base, an output signal characterizing a switching operation can be tapped from their collector, and their emitter is set to a predetermined potential. A switching operation always occurs when the input signal reaches a voltage corresponding to the base-emitter voltage compared to the predetermined potential.Such circuit arrangements for evaluating the output signal of inductive or capacitive proximity switches are, in other words, comparators for comparing the input signal, which corresponds to the demodulated signal of such a sensor device, and the reference signal, which corresponds to the predetermined potential at the emitter of the transistor.

[0011] Electromagnetic interference, which can particularly affect the oscillator, is now problematic. Sensor devices featuring proximity switches of this type may, in some cases, fail to meet the requirements of the required EMC (electromagnetic compatibility) standards. Problems arise, for example, in the case of electrostatic discharge (ESD). Such electrostatic discharges can, for example, interrupt the oscillations in the oscillator, triggering a switching operation without the object to be detected approaching the proximity switch.

[0012] The invention is based on the object of improving a circuit arrangement and a sensor device as described above in such a way that electromagnetic interference does not influence the switching signal, in particular that electrostatic discharges do not trigger unwanted switching operations and that the EMC compatibility is improved overall. Disclosure of the invention

[0013] The problem is solved by a circuit arrangement for evaluating the output signal of an inductive or capacitive proximity switch for switching operations in the low to medium frequency range with a transistor in emitter circuit, to whose base an input signal can be fed, at whose collector an output signal characterizing a switching operation can be tapped and whose emitter is set to a predetermined potential, in that a coupling capacitor is connected between emitter and base, which transmits disturbing short-term voltage peaks at the base directly to the emitter.

[0014] In other words, this coupling capacitor suppresses short-term voltage spikes that could lead to unwanted switching by raising the emitter potential in the same way as the input signal at the transistor's base. Common-mode noise is not affected by this circuit.

[0015] According to one aspect of the invention, the emitter is set to the predetermined potential by a voltage divider.

[0016] The low to medium frequency range of the switching operations is preferably between 50 Hz and 1.5 KHz, in particular between 100 Hz and 1 KHz.

[0017] The sensor device according to the invention with an inductive or capacitive proximity switch and an evaluation circuit which comprises an oscillator, a demodulator for outputting a demodulated signal, and a comparator for comparing the demodulated signal with a reference signal and for outputting an output signal depending on the comparison, is characterized in that the comparator is realized by a circuit arrangement as described above, wherein the demodulated signal corresponds to the input signal at the base of the transistor and the reference signal corresponds to the predetermined potential at the emitter of the transistor and wherein the coupling capacitor couples high-frequency interference of the demodulated signal to the reference signal.

[0018] Such high-frequency interference includes, for example, voltage spikes caused by electrostatic discharge. For the reasons mentioned above, coupling the high-frequency interference of the modulated signal to the reference signal prevents unwanted switching operations that signal the approach of, for example, a metallic object to the proximity switch. Short description of the drawings

[0019] Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description. Fig. 1 shows a schematic representation of a sensor device known from the prior art. Fig. 2 shows a schematic representation of a sensor device according to the invention. Fig. 3 shows a circuit arrangement known from the prior art for evaluating the output signal of an inductive or capacitive proximity switch. Fig. 4 shows a circuit arrangement according to the invention for evaluating the output signal of an inductive or capacitive proximity switch. Embodiments of the invention

[0020] A sensor device known from the prior art for an inductive or capacitive proximity switch 105 has an evaluation circuit 100 which comprises an oscillator 110, a demodulator 120 for outputting a demodulated signal Vdem and a comparator 130 for comparing the demodulated signal Vdem with a reference signal VRef and for outputting an output signal Vs depending on the comparison.

[0021] The comparator 130 is controlled by a Fig. 3, is realized, known from the prior art, for evaluating the output signal of the proximity switch, i.e., the demodulated signal Vdem. This circuit arrangement comprises a bipolar transistor T in a common emitter circuit, the emitter of which is set by a voltage divider R2, R3 to a predetermined potential relative to the supply voltage Vcc, in this case the reference signal or the reference voltage VRef. The input signal can be fed to the base of the transistor T via a resistor R1, which in the circuit arrangement according to Fig. 1 corresponds to the demodulated signal or the demodulated voltage Vdem output by the demodulator 120. The output signal or the output voltage Vs is applied to a resistor R4 connected downstream of the collector and is fed, for example, to a further amplifier stage (not shown) and evaluated. A switching operation always takes place when the demodulated signal Vdem corresponds to the reference signal VRef plus the base-emitter voltage of the transistor T. By appropriately wiring the transistor T and the other Fig. A desired switching distance can be set using the switching elements shown in Figure 1.

[0022] If high-frequency interference is present at the sensor device and thus the circuit device, this can lead to the demodulated signal Vdem being briefly increased without a metallic object approaching the (inductive) proximity switch 105. In this case, a switching operation takes place and the output signal Vs falsely signals the approach and presence of a metallic object in the detection range of the proximity switch 105. Such high-frequency interference can occur, for example, due to electrostatic discharges or other electromagnetic influences. In order to eliminate these, a sensor device according to the invention, which is Fig. 2 and in which the same elements are given the same reference numerals as in Fig. 1 are provided with a coupling capacitor CK, which couples high-frequency interference from the demodulated signal Vdem to the reference signal VRef. If, for example, the demodulated signal Vdem is boosted due to such high-frequency interference, the coupling capacitor CK simultaneously boosts the reference signal VRef in the same way, thus preventing a switching operation. This effectively prevents the influence of high-frequency interference. Common-mode interference, on the other hand, does not affect the circuit. The coupling capacitor CK thus "couples" the input signal, the demodulated signal Vdem, to the reference signal VRef.

[0023] The coupling capacitor CK is connected between the emitter potential corresponding to the reference signal VRef and the input signal corresponding to the demodulated signal Vdem, as in the circuit arrangement according to the invention for evaluating the output signal, shown in Fig. 4, in which the same reference numerals are used as in Fig. 3. In this way, the aforementioned high-frequency interference present at the base of transistor T is transmitted directly to the emitter of transistor T, whereby the emitter potential and thus the reference signal VRef is "raised" in the case of high-frequency interference in the same way as the input signal present at the base, i.e., the demodulated signal Vdem.

[0024] The advantage of this simple circuitry is that it avoids complex modifications to the demodulator, such as changing the cutoff frequency of the demodulator implemented as a low-pass filter or re-dimensioning the oscillator and sensor element. These modifications would result in a complex circuit design, increased complexity, and increased development and manufacturing costs. The easy-to-implement coupling capacitor CK, on ​​the other hand, provides a cost-effective and effective solution that avoids the EMC compatibility of such sensor devices and, in particular, insensitivity to electrostatic discharges (GSD).

Claims

[1] Circuit arrangement (10) for evaluating the output signal of an inductive or capacitive proximity switch (105) for switching operations in the low to medium frequency range, comprising a transistor (T) in common emitter circuit, to the base of which an input signal (Vdem) can be fed, at the collector of which an output signal (Vs) characterising a switching operation can be tapped and the emitter of which is set to a predetermined potential (VRef), characterized by that a coupling capacitor (Ck) is connected between the emitter and the base, which transfers disturbing short-term voltage peaks at the base directly to the emitter. [2] Circuit arrangement according to claim 1, characterized by that the emitter is set to the specified potential by a voltage divider. [3] Circuit arrangement according to claim 1, characterized by that the low to medium frequency range is between 50 Hz and 1.5 KHz, especially between 100 Hz and 1 KHz. [4] Sensor device comprising an inductive or capacitive proximity switch (105) and an evaluation circuit (100) which comprises an oscillator (110), a demodulator (120) for outputting a demodulated signal (Vdem), and a comparator (130) for comparing the demodulated signal (Vdem) with a reference signal (VRef) and for outputting an output signal (Vs) depending on the comparison, characterized by that the comparator (130) is realized by a circuit arrangement (10) according to one of claims 1-3, wherein the demodulated signal (Vdem) corresponds to the input signal at the base of the transistor (T) and the reference signal (VRef) corresponds to the predetermined potential at the emitter of the transistor (T), and wherein the coupling capacitor (Ck) couples high-frequency interference of the demodulated signal (Vdem) to the reference signal (VRef).

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